Neuroglandular synapses in the pharynx of the sea anemone Aiptasia pallida (Cnidaria, Anthozoa).

Scanning and transmission electron microscopy of the pharynx of the sea anemone Aiptasia pallida revealed a heavily ciliated epidermis and two types of gland cells not known previously to be innervated. By tracing serial cross sections of the pharynx, we located and characterized two types of neuroglandular synapses (i.e., those having clear vesicles and those with dense-cored vesicles). The diameters of the vesicles at each synapse were averaged; clear vesicles ranged from 70 to 103 nm in diameter and were observed at synapses to both mucous and zymogenic gland cells. Dense-cored vesicles ranged from 53 to 85 nm in diameter and were observed at synapses to two mucous gland cells. One mucous gland cell had three neuroglandular synapses, one with clear vesicles and two with dense-cored vesicles. The occurrence of either clear or dense-cored vesicles at neuroglandular synapses suggests that at least two types of neurotransmitter substances control the secretion of mucus in the sea anemone pharynx. To date, only clear vesicles have been observed at a neurozymogenic gland cell synapse in the pharynx. No evidence of immunoreactivity to phenylethanolamine-N-methyl transferase was observed at neuroglandular synapses, suggesting that adrenaline is not a transmitter in the pharynx of A. pallida.

Interalveolar septal changes with Pasteurella haemolytica-induced pneumonia in exertion-stressed calves.

This study compared the effects of exertion stress and exertion stress combined with P. haemolytica infection on cells in the pulmonary capillaries and interalveolar septa adjacent to pneumonic lung. Two calves stood on an Anamill treadmill to serve as nonexercised controls and four calves were run to exhaustion (exertion-stressed) on the treadmill. Two of these four calves were inoculated endobronchially with 5 x 10(9) CFU of virulent Pasteurella haemolytica immediately following treadmill exercise. All calves were euthanized 24 h later and lung tissue was collected from the right mid-caudal lung lobe, next to the area of the gross lesion in inoculated calves. Ultrastructural evidence of increased metabolic activity of pulmonary intravascular macrophages (PIMs) and slight alveolar edema were present after exertion stress, but no changes in numbers of PIMs and platelets were observed between exertion-stressed and control calves. Lungs of calves that were subjected to stress and experimental infection had increases in size, number, and metabolic activity of PIMs, and numerous platelets that were interspersed among the PIMs. These findings suggest that exercise has minimal effect on the cellular changes in interalveolar septa after 24 h. However, exercise with P. haemolytica infection can stimulate PIM activity at short distances (<2.5 mm) from the gross pneumonic lesion. It is possible that the alterations in PIMs are stimulated by cytotoxic products arising from a nearby lesion in which bacteria and infiltrated cells are present.

Comparative ultrastructure of Clara cells in neonatal and older cattle.

Calf lungs were fixed with glutaraldehyde and examined by scanning (SEM) and transmission (TEM) electron microscopy to compare the ultrastructure of Clara cells in terminal bronchioles of neonatal calves and older cattle. In the neonatal calf, SEM revealed numerous smooth-surfaced Clara cells protruding above a similar number of ciliated cells, whereas in older animals the surface of Clara cells was lobulated. Thin sections examined by TEM revealed numerous cuboidal to columnar Clara cells with indented nuclei and a pale cytoplasm filled with faintly granular glycogen in the neonatal calf. Some cells were characterized by apical dense and/or pale membrane-bound granules or secretory droplets. Many cells had an apical tubular network of cisternae that were partly smooth and partly decorated with ribosomes. Ultrastructural comparison of Clara cells in a 2-day-old calf with those of 14- and 19-day-old, 4- and 5. 5-month-old, and 3.5-year-old cattle revealed a striking reduction in the amount of glycogen per cell after 14 days. The number of cells with apical granules was small at all ages, and the density of the secretory granules varied greatly in different cells. A variable amount of smooth endoplasmic reticulum (SER) was present but was less prominent than cisternae of ribosomal endoplasmic reticulum (RER). In older cattle, the limited amount of SER compared to the RER and secretory granules suggests that bovine Clara cells are more likely to be secretory than detoxifying.

Ultrastructure of neuro-spirocyte synapses in the sea anemone Aiptasia pallida (Cnidaria, Anthozoa, Zoantharia).

Using transmission electron microscopy of serially sectioned tentacles from the sea anemone Aiptasia pallida, we located and characterized two types of neuro-spirocyte synapses. Clear vesicles were observed at 10 synapses and dense-cored vesicles at five synapses. The diameters of vesicles at each neuro-spirocyte synapse were averaged; clear vesicles ranged from 49-89 nm in diameter, whereas the dense-cored vesicles ranged from 97-120 nm in diameter. One sequential pair of synapses included a neuro-spirocyte synapse with clear vesicles (81 nm) and a neuro-neuronal synapse with dense-cored vesicles (168 nm). A second synapse on the same cell had dense-cored vesicles (103 nm). An Antho-RFamide-labeled ganglion cell and three different neurites were observed adjacent to spirocytes, but no neuro-spirocyte synapses were present. Many of the spirocytes also were immunoreactive to Antho-RFamide. The presence of sequential neuro-neuro-spirocyte synapses suggests that synaptic modulation may be involved in the neural control of spirocyst discharge. The occurrence of either dense-cored or clear vesicles at neuro-spirocyte synapses suggests that at least two types of neurotransmitter substances control the discharge of spirocysts in sea anemones.

Ultrastructure of Pasteurella haemolytica-induced changes in gnotobiotic calf lung.

Gnotobiotic calves born and maintained in a germ-free environment until inoculated with a pathogen are model animals for studying the progression of a specific disease, such as pneumonic pasteurellosis. To investigate early progression of pneumonic pasteurellosis, we compared the ultrastructure of regions of gas-exchange in the lungs of three challenge-exposed and three uninoculated control gnotobiotic calves. Three calves were inoculated endobronchially with a bolus of 7.9 x 10(10) CFU of Pasteurella haemolytica A1 and studied in a specific pathogen-free environment until severe respiratory distress occurred, at which time they were euthanized. Slices of lung tissue from the midregion of the right dorsal caudal lobe (area of lesion) of infected and control calves were fixed in glutaraldehyde and prepared for scanning (SEM) and transmission (TEM) electron microscopy. SEM revealed bacteria among long tangled strands of fibrin in pulmonary alveoli that became obliterated with cellular debris. TEM revealed areas of encapsulated and/or nonencapsulated bacteria among the cellular debris and patches of fibrin. Many neutrophils and macrophages that infiltrated the alveoli had phagocytosed bacteria and undergone degradation. Less cellular damage was present when encapsulated bacteria bordered the interalveolar septa than when nonencapsulated lysed bacteria were present. Where lysed bacteria were present, the pulmonary capillaries were dilated because of swollen, degranulated neutrophils, fibrin clots, and cellular necrosis. Both encapsulated and nonencapsulated bacteria were present in the lung tissue of gnotobiotic calves within the first 24 h after endobronchial inoculation of early log phase P. haemolytica. Loss of capsular material around individual and divided pairs of bacteria led to their consequential aggregation, lysing, and severe damage to the adjacent pulmonary capillaries and interalveolar septa.

Ultrastructure of neurons and synapses in the tentacle epidermis of the sea anemone Calliactis parasitica.

The anatomical organization of neutrons and synaptic pathways in tentacles of sea anemones is poorly understood. Transmission electron microscopy of serial thin sections was carried out on various regions of tentacles of the sea anemone Calliactis parasitica in order to locate and characterize typical epidermal neutrons and synapses. Both surface-oriented sensory cells with ciliary cones and basally located ganglion cells lacking a cilium have Golgi-derived granular or faintly cored vesicles. Similar vesicles are present at synaptic loci on some ganglion and muscle cells. The synaptic contacts on the longitudinal muscle cells are generally en passant rather than terminal. They vary from single neuromuscular synapses to pairs of neurites innervating the same muscle cell or one neurite innervating two or more muscle cells. Both two-way and one-way interneuronal synapses with vesicles aligned at paired synaptic membranes with dense material in a 14-20-nm-wide cleft are present in the epidermal nerve plexus. The vesicles average from 50 to 80 nm in diameter and vary from electron lucent to faintly cored. The results of this study demonstrate the presence of a complex system of epidermal neuronal pathways with specific synaptic loci in this modern representative of a first-evolved nervous system.

Exercise-induced changes in the lung of Shetland ponies: ultrastructure and morphometry.

The ultrastructural changes in pulmonary alveoli produced by running two ponies on a high speed treadmill at 7.6 m/sec, 3-degree incline, for 2 min support the hypothesis of pulmonary capillary stress failure as an explanation for exercise-induced pulmonary hemorrhage (EIPH). Light microscopy combined with scanning and transmission electron microscopy confirmed the presence of red blood cells and proteinaceous material in the alveolar lumina and interstitial swelling in approximately one third of the pulmonary alveoli examined. Morphometric analysis revealed that the blood-gas barrier was 30-77% thicker on the thin respiratory surface of the interalveolar septa in the cranial lobe of the two exercised ponies, i.e., 0.62 and 0.46 micron, compared to that of the unexercised control pony (0.35 micron). No change in blood-gas barrier thickness was observed in the caudal lobe, although that is where EIPH lesions have been observed in race-horses. Vascular pressures were low (20 mm Hg) in the pulmonary circulation of the Shetland pony at rest but increased more than three fold to 63 mm Hg during exercise. These preliminary morphological and physiological results indicate that a short burst of near-maximal exercise in a non-athletic equine can lead to structural changes in the blood-gas barrier and leakage of blood from pulmonary capillaries despite pulmonary vascular pressures being significantly lower than previously found in the racehorse.

Classification of Nerve Cells Dissociated From Tentacles of the Sea Anemone Calliactis parasitica.

Nerve cells from tentacles of the sea anemone Calliactis parasitica were dissociated in 1000 units/ml of collagenase for scanning electron microscopic studies and in 0.125% elastase followed by 12 units/ml of ficin for cell counts using light microscopy. The studies revealed 33 distinguishable neuronal shapes, which were categorized as either sensory cells having an apical cilium or ganglion cells with or without a perikaryal cilium and further subdivided into unipolar, bipolar and multipolar neurons based on the number of processes that extended from the perikaryon. Unipolar sensory cells were characterized by an apical cilium adjacent to the perikaryon and a long, simple or terminally branched axon. Unipolar ganglion cells lacked an apical cilium. Bipolar sensory cells had a neck between the cilium and perikaryon and an oppositely directed axon. Bipolar ganglion cells had isopolar processes or asymmetrical processes, which were simple or complexly branched. One type of bipolar ganglion cell with isopolar processes had a perikaryal cilium. Multipolar sensory cells had a distinct neck between the perikaryon and the cilium and two or more simple or complexly branched processes extending from the triangularly shaped cell body. Multipolar ganglion cells had variously shaped perikarya from which extended three or more short or long processes that were simple or complexly branched. One type of tripolar ganglion cell had a perikaryal cilium. The different types of nerve cells were quantified, and statistical comparisons were made.

The nervous systems of cnidarians.

Cnidarians have simple nervous systems and it was probably within this group or a closely-related ancestor that nervous systems first evolved. The basic plan of the cnidarian nervous system is that of a nerve net which, at some locations, has condensed to form nerve plexuses, or circular or longitudinal nerve tracts which may be syncytia. At the ultrastructural level, many cnidarian neurons have the combined characteristics of sensory, motor, inter- and neurosecretory neurons and thus appear to be multifunctional. We propose that these multifunctional neurons resemble the ancestors of the more specialized neurons that we find in higher animals today. The primitive nervous system of cnidarians is strongly peptidergic: from a single sea anemone species Anthopleura elegantissima, we have now isolated 16 different novel neuropeptides. These peptides are biologically active and cause inhibitions or contractions in muscle preparations or isolated muscle cells from sea anemones. The various peptides are located in at least six distinct sets of neurons showing that sea anemone neurons have already specialized with respect to their peptide content. Using immuno-electronmicroscopy, we have found that the peptides are located in neuronal dense-cored vesicles associated with both synaptic and non-synaptic release sites. All these data indicate that evolutionarily "old" nervous systems use peptides as transmitters. We have also investigated the biosynthesis of the cnidarian neuropeptides. These neuropeptides are made as large precursor proteins which contain multiple (up to 36) copies of immature neuropeptides. Thus, the biosynthesis of neuropeptides in cnidarians is very efficient and comparable to that of higher invertebrates, such as molluscs and insects, and vertebrates.

Ultrastructural Localization of Antho-RWamides I and II at Neuromuscular Synapses in the Gastrodermis and Oral Sphincter Muscle of the Sea Anemone Calliactis parasitica.

Light microscopic studies have shown that the sea anemone neuropeptides Antho-RWamides I (

Actinobacillus pleuropneumoniae in gnotobiotic piglets: ultrastructural changes in the pulmonary alveoli with dose and time.

The ultrastructural changes in pulmonary alveoli produced by transtracheal inoculation of 10(6) and 10(8) CFU of Actinobacillus pleuropneumoniae serotype 5 in gnotobiotic piglets were studied after 1 and 4 h. At 1 h postinoculation (p.i.) with 10(6) or 10(8) CFU, no gross change in lung and no evidence of infiltration of cells into alveoli was observed. At 4 h p.i., at a dose of 10(6) CFU, a generalized red mottling occurred in the dorsal half of the caudal lobe, which revealed ultrastructural evidence of neutrophil infiltration into alveoli along with fibrin and a few erythrocytes. At 4 h p.i., at a dose of 10(8) CFU, there was a bilateral lung lesion characterized by a generalized mottling and congestion, within which we observed ultrastructural evidence of bacteria and cellular debris in the alveoli and fibrin clots and cellular necrosis in the alveolar septum. By using gnotobiotic piglets and visualizing the effects of bacterial inoculum on cellular ultrastructure of the lung, we have demonstrated that both dose and time play roles in the early pathogenesis of experimental porcine pleuropneumonia. The developing lesion in lungs of gnotobiotic piglets infected with pure cultures of A. pleuropneumoniae can be controlled experimentally. This experimental procedure can provide a base of reproducible, sequential, ultrastructural changes with which to compare the role of inflammatory mediators in the lung and the effects of drugs on immunologic events in the lung.

Antho-RFamide Immunoreactivity in Neuronal Synaptic and Nonsynaptic Vesicles of Sea Anemones.

Antho-RFamide is a neuropeptide isolated from the sea anemone Anthopleura elegantissima. Antho-RFamide immunoreactivity was localized in four different populations of neuronal vesicles in the tentacle nerve plexus of Anthopleura. Small, opaque, neuronal vesicles, averaging 49 nm in diameter, were gold-labeled at two-way synapses. Heterogranular vesicles, averaging 184 nm in diameter, were gold-labeled in a neuronal swelling adjacent to a muscle cell process. These vesicles were similar in size to a third class of gold-labeled dense-cored vesicles. A fourth class of immunogold-labeled vesicles observed in neuronal swellings had light cores and averaged 129 nm in diameter. Using 5-nm gold particles, we observed a heavy labeling of the granular cores of the dense-cored vesicles, suggesting that the immunoreactivity is specific to the vesicle core. The ultrastructural demonstration of Antho-RFamide immunoreactivity in interneuronal synaptic vesicles, together with the immunofluorescence and electrophysiological studies of other investigators, suggest that Antho-RFamide plays a role in neurotransmission in sea anemones.

Multifunctional features of a gastrodermal sensory cell in Hydra: three-dimensional study.

Computer-assisted, three-dimensional reconstructions of two gastrodermal sensory cells from transmission electron micrographs of serial sections of Hydra revealed a unipolar morphology with the nucleus near an apical cilium and a simple unbranched axon with a widened terminal. The sensory cells were similar in size and shape to a unipolar sensory cell isolated from macerated gastrodermis and examined with scanning electron microscopy. In thin sections, the cells were characterized by the presence of numerous dense-cored vesicles in the axon and its terminal. A few dense-cored vesicles were aligned at electron-dense synaptic foci in the axon terminal of the sensory cell, which formed an axo-axonal synapse with a nearby centrally located ganglion cell and a neuromuscular synapse with the basal myoneme of a digestive cell. The ganglion cell possessed a perikaryal cilium and a slender axon that extended adjacent to the sensory cell terminal, where it formed an en passant axo-axonal synapse in reciprocal arrangement with that of the sensory cell. In addition, the ganglion cell axon formed a neuromuscular synapse in sequence with the sensory cell axo-axonal synapse. The presence of a large number of neurosecretory-like granules, apical cilium and reciprocal interneuronal and neuromuscular synaptic loci suggests that this gastrodermal sensory cell, characterized ultrastructurally for the first time, represents a third type of multifunctional neuron in Hydra. Thus, Hydra may contain primitive stem-like neurons, which are sensory-motor and also function in both neurosecretion and neurotransmission.

Preservation of tracheal mucus by nonaqueous fixative.

Two nonaqueous fixatives, composed of fluorocarbon solvents with dissolved osmium tetroxide, were used to determine the feasibility of preserving the mucous coat in bovine and rat trachea for light and electron microscopy. Aqueous fixatives, while providing excellent cytological preservation, wash away the mucous lining, precluding ultrastructural analysis. Inclusion of ruthenium red or alcian blue within aqueous fixative improved retention of mucus, but provided incomplete, patchy results. Fixation with nonaqueous fluorocarbon solvent and dissolved osmium tetroxide preserved a continuous mucous epiphase layer above a clear hypophase layer. Subcomponents of the mucus included an electron dense surface layer, interrupted patches of mucus above the surface layer and electron dense membrane-like material within the mucus. This method of fixation will preserve mucus for light, scanning and transmission electron microscopy, using either intratracheal or immersion methods of fixation. The latter would enable use of materials from large animal models, autopsy or an abattoir.

Lysosomal interconnections and horseradish peroxidase compartmentalization in three-dimensionally reconstructed bovine alveolar macrophages.

Following a 1-h incubation of bovine alveolar macrophages in 1 to 2 mg/ml exogenous horseradish peroxidase (HRP), ultrathin sections revealed vacuolar interconnections among both labeled and unlabeled vacuoles constituting the lysosomal compartment. Four entire cells and their vacuolar components were subsequently computer resconstructed from serial transmission electron micrographs and measured using a morphometric technique. HRP-labeled and unlabeled vacuoles ranged in size from 0.5 micron to greater than or equal to 4.0 microns in diameter and occupied up to 25% of the cytoplasmic volume. HRP-containing vacuoles were distributed throughout each cell in a clumped distribution (P less than 0.05) and occupied up to 75% of the total vacuole compartment. Up to 60% of all vacuoles were interconnected through a series of openings formed by membrane fusions (average pore diameter 0.42 micron), which resulted in a labyrinth of vacuoles comprising up to 55% of the total volume of the lysosomal compartment. The area of open interconnections resulting from vacuolar fusions represented less than 1% of the total surface area of the lysosomal membrane. Rotation of a three-dimensionally reconstructed macrophage about the Y-axis revealed an interconnected vacuolar network of 75 fused vacuoles in a chain up to 21 microns in length. We have demonstrated that HRP-labeled vacuoles interconnect with each other as well as with preexisting unlabeled vacuoles. As a result of such interconnections, individual vacuoles become contributing members of a large, continuous, lysosomal compartment in bovine alveolar macrophages.

A combined high-voltage and scanning electron microscopic study of two types of sensory cells dissociated from the gastrodermis of Hydra.

Sensory cells dissociated from gastrodermis of Hydra were studied using high-voltage electron microscopy (HVEM) for visualization of internal features and scanning electron microscopy (SEM) to clarify surface details. HVEM revealed whole cell morphology, nuclear shape and location, apical ciliary-microvillar organization, length of microtubule bundles, and distribution of dense-cored vesicles in axonal processes and nerve terminals. SEM of the same cells revealed surface blebs and membrane perforations not easily recognized by HVEM alone. Hence, mapping of SEM surface structures can be used to determine what is surface versus interior structures in the HVEM image and thus aid in interpreting such images. The maceration procedure revealed a previously undescribed, pyriform-shaped sensory cell with oppositely-directed basal processes. This combined HVEM-SEM study of whole cells provided evidence of possible synaptic loci in an axon terminal and demonstrated a new morphological type of gastrodermal sensory cell in Hydra. These early-evolved sensory cells appear to be sensory-motor with neurosecretory granules.

Ultrastructural localization of RFamide-like peptides in neuronal dense-cored vesicles in the peduncle of Hydra.

The presence of Arg-Phe-amide (RFamide)-like peptides in dense-cored vesicles in neurons of the peduncle of Hydra was demonstrated by immunogold electron microscopy. Thin sections of Lowicryl-embedded tissue labeled with antisera to RFamide and 5-nm gold-conjugated, secondary antibody and of Epon-Araldite-embedded tissue labeled with 15-nm gold particles revealed a concentration of RFamide-like immunoreactivity over the granular cores of vesicles in epidermal ganglion cells. Gold-labeled, dense-cored vesicles were present in the perikaryon, long thin neurites, and axon terminals of these neurons. The aggregation of labeled dense-cored vesicles in an axon terminal on the myoneme of an epitheliomuscular cell suggests a possible function of RFamide-like peptides in neuromuscular transmission. Gold staining of dense-cored vesicles completely disappeared when the RFamide antiserum was preabsorbed with 10 micrograms/ml RFamide. These results are the first demonstration that the dense-cored vesicles of Hydra neurons contain a neuropeptide.

Use of a monoclonal antibody to classify neurons isolated from the head region of Hydra.

A mouse monoclonal antibody (JD1) to Hydra attenuata using the peroxidase-antiperoxidase (PAP) method revealed unipolar, bipolar, and multipolar sensory and ganglion cells in the head region of H. littoralis. Neurons isolated from macerated hypostomes and tentacles were classified according to the number of their cytoplasmic processes and the position of the cilium, when present, relative to the perikaryon. PAP-stained sensory cells had an apical ciliary cone, whereas ganglion cells did not. Neurons with cytoplasmic processes longer than 50 microns stained faintly, whereas those with processes shorter than 50 microns in length stained mainly dense brown. Unipolar neurons had an oval, crescent, round, or elliptic perikaryon with a single short axon. The perikaryal shape of bipolar neurons varied from round to tall triangular, short triangular, crescent, oval, or elliptic with two oppositely directed symmetric or asymmetric processes. Asymmetric processes were present in a bipolar sensory cell with a long apical cilium typical of gastrodermal sensory cells. One type of bipolar ganglion cell had a short perikaryal cilium. Another type had neurites longer than 50 microns. We found seven morphological variations of multipolar neurons, including one with an apical knob, two with a short perikaryal cilium, two with cytoplasmic loops near the perikaryon, one with perpendicular processes projecting from the major neurites, and one with a branched process longer than 50 microns opposite a tangled mass of neurites.

Ultrastructural alterations of motor cortex synaptic junctions in Brown Swiss cattle with weaver syndrome.

An ultrastructural comparison was made between the motor cortex synapses of the brain of 4 adult Brown Swiss cattle with weaver syndrome and 2 healthy cattle (a Holstein steer and a Guernsey cow). Compared with the healthy cattle, the paramembranous densities at synaptic junctions of cattle with weaver syndrome had a decreased mean height of the presynaptic dense projections (P less than 0.001), had significantly smaller peak-to-peak distances (P less than 0.001), and had a significantly decreased mean thickness of the postsynaptic density (P less than 0.0001). These synaptic changes in cattle with weaver syndrome may be associated with clinical manifestations of the syndrome, such as impairment of transmitter release, instability of postsynaptic receptor sites, lesion-induced synaptic turnover from injury elsewhere in the CNS, or loss of a specific cell population of the motor cortex.

Light and electron microscopic localization of a monoclonal antibody in neurons in situ in the head region of Hydra.

A mouse monoclonal antibody to Hydra attenuata was used to demonstrate immunoreactive product in neurons in situ, in both whole mount and sectioned hypostomes and tentacles of H. oligactis and H. littoralis. Immunoreactive cells were concentrated around the mouth and scattered along the length of the tentacles. In the hypostome, nerve cells sent one or more processes orally and the others aborally but the processes were more distinctly stained in H. oligactis. A thin strand of five to six perihypostomal neurons was present close to the hypostome-tentacle junction. In the tentacles, neurons with long processes contacted up to five different batteries of nematocysts. Neural processes were associated with nematocyst batteries in three ways: 1) forming a perikaryal loop to encircle a centrally located stenotele, 2) branching at a distance from the perikaryon to contact a variety of nematocysts, and 3) terminal branching by one or more neurons with contacts on one to several nematocysts within a battery. Immunocytochemical localization of neurons in Hydra by light microscopy was correlated for the first time with electron microscopy. Peroxidase-antiperoxidase (PAP)-positive sensory cells were concentrated around the mouth opening. PAP-positive ganglion cells were predominant in the tentacles. Sensory cells were elongate or spindle-shaped (unipolar), triangular with two oppositely directed processes (bipolar), and multipolar (tripolar or tetrapolar) with one of the processes extending to the epidermal surface. Ganglion cells were either unipolar or bipolar or multipolar, with neurites paralleling the mesoglea and occasionally having processes abut on it.