Original molluscan radula: comparisons among Aplacophora, Polyplacophora, Gastropoda, and the Cambrian fossil Wiwaxia corrugata.

As the original molluscan radula is not known from direct observation, we consider what the form of the original radula may have been from evidence provided by neomenioid Aplacophora (Solenogastres), Gastropoda, Polyplacophora, and the Cambrian fossil Wiwaxia corrugata (Matthews). Conclusions are based on direct observation of radula morphology and its accessory structures (salivary gland ducts, radular sac, anteroventral radular pocket) in 25 species and 16 genera of Aplacophora; radula morphogenesis in Aplacophora; earliest tooth formation in Gastropoda (14 species among Prosobranchia, Opisthobranchia, and Pulmonata); earliest tooth formation in four species of Polyplacophora; and the morphology of the feeding apparatus in W. corrugata. The existence of a true radula membrane and of membranoblasts and odontoblasts in neomenioids indicates that morphogenesis of the aplacophoran radula is homologous to that in other radulate Mollusca. We conclude from p redness of salivary gland ducts, a divided radular sac, and a pair of anteroventral pockets that the plesiomorphic state in neomenioids is bipartite, formed of denticulate bars that are distichous (two teeth per row) on a partially divided or fused radula membrane with the largest denticles lateral, as occurs in the genus Helicoradomenia. The tooth morphology in Helicoradomenia is similar to the feeding apparatus in W. corrugata. We show that distichy also occurs during early development in several species of gastropods and polyplacophorans. Through the rejection of the null hypothesis that the earliest radula was unipartite and had no radula membrane, we conclude that the original molluscan radula was similar to the radula found in Helicoradomena species.

[Radula replacement and cell proliferation in the X-irradiated radular gland of the slugLimax flavus L. On the function of the radular gland epithelia]. / Radulaersatz und Zellproliferation in der röntgenbestrahlten Radulascheide der NacktschneckeLimax flavus L. Ergebnisse zur Arbeitsteilung der Scheidengewebe.

Investigations have been carried out on the long-term effect of a single whole body X-irradiation on radula, radula replacement rate, and radular gland ofLimax flavus L. 1. Damage in the radula. In the course of 8 weeks after irradiation with 50 000 R two separate damaged areas develop in the radula. Immediately after exposure 1-2 transverse rows of defect teeth arise. Posterior to an area with normal transverse rows an extensive zone of malformed teeth develops from the 2nd week on. Normal transverse rows are produced again after the 8th week. 2. Replacement rate. Adult snails replace 3-3,5 transverse rows/day during 5 weeks after a dose of 40250 R. The replacement rate decreases to 1,1 rows/day in the 6th week (Fig. 3, broken-lined graph). 3. Damage in the radular gland epithelia. After irradiation with 50 000 R the proliferation zones of thesuperior andinferior epithelium differ with respect to the extent of damage. In the first mentioned area numerous cells die; the cell proliferation is strongly reduced for weeks and reaches a normal level again at the beginning of the 10th week after exposure. The superior epithelium and its proliferation zone are temporary atrophied (Fig. 5a, b and 7a, b). They have recovered in the 10th week. The mitotic activity in theinferior epithelium is less reduced than in the proliferation zone of the superior epithelium. It has almost normalized in the 5th week after exposure. Only a few inferior epithelium cells die; atrophy of the inferior epithelium does not occur. From the 3rd week on after exposure the odontoblasts in numerous radular glands are deformed (Fig. 8). They assume their normal shape again in the beginning of the 10th week. No odontoblasts die after the irradiation. 4. CONTROLS: Head-shielded snails were irradiation with 50 000 R for controls. No effects of the body irradiation were found in the radula or in the radular gland epithelia. 5. Division of labour in the radular gland. The temporary elimination of the superior epithelium and the epithelial region above the odontoblasts-groups does not affect the tooth-formation and the radula transport into the oral cavity. Hence it follows: a) The odontoblasts-group is exclusively responsible for the definitive shape of the tooth (Fig. 8 and 9). b) The radula is transported into the oral cavity by the inferior epithelium (cf. Chap. E, II, b). On account of these results it is possible for the first time to described comprehensively the division of labour in the radular gland (cf. Chap. E, V). 6. Development of abnormal radular gland epithelia. After irradiation with 50 000 R an abnormal epithelial system develops from the 2nd week on in the tip of the radular gland (Fig. 10a, b;aEep). It encloses sphaeric or tube-like cavities. In many cases they communicate with the lumen of the radular gland. The light-microscopic appearence of the cells of this abnormal epithelium resembles those at the tip of a non-irradiated radular gland (Fig. 11 a, b).

[Investigations into the X-ray-influenced radula-replacement in the naked snail Limax flavus L.] / Untersuchungen mittels Röntgenbestrahlung über den Radula-Ersatz der NacktschneckeLimax flavus L.

1. The naked snailLimax flavus L. has a radula with longitudinal rows of uniform teeth and cross-rows of teeth with a gradually changing form towards the radula's edge in regular succession, (Fig. 1). The individual tooth sits on the front part of a basal plate. This lies on the radula membrane (Fig. 2). 2. The radula is constantly shortened at its frontal end and is continuously renewed at the rear end of the radula gland. - This is an epidermal fold of the mouth cavity. At this place of radula-development lies a region of large secretion-cells, the odontoblasts. These are arranged together in groups (Fig. 10, 11 and 13). Each longitudinal toothrow with the underlying membrane, is produced by one group of odontoblasts. Each group for a lateral tooth in the medial region of the radula consists of 15 cells. During the substitution of the cross-row processes of developing teeth in the medial part get more and more ahead of those at the edge. 3. In an odontoblasts-group one finds division of labour. The front cells produce membrane material and the basal plate. The rear cells of the group produce the tooth. During the formation of the tooth the rear cells change their apical surface profile. The form of the tooth's upperside corresponds to the apical profile of cells at the beginning of secretion (Fig. 14a, 15a, b). The form of the tooth's furrow responds to that of the apical odontoblasts-profile at the end of secretion (Fig. 14b, 15c). The odontoblasts are exclusively responsible for the definitive shape of the tooth. 4. After a single whole-body X-irradiation of young snails (dosis-range: 8,050-130,000 R, Table 2) a characteristically changed pattern of teeth arises. A region of cross-rows, following each other closer than normal, extends across the radula (Fig. 4, 80,500 R). The condensation of cross-rows depends on the dosis (Fig. 8, broken-lined graph). Slides show an abnormal form and irregular position of teeth and their basal plate (comp. Fig. 2a, b and 17). 5. The radula replacement system shows great resistance against X-rays. Up to a dosis of 113,000 R it is able to recover. It can again develop more or less normally formed and situated teeth and basal plates. Only after a dosis of 130,000 R (LD100:6 days, Table 2) odontoblasts do not recover, although they produce up to 9 cross-rows before the death of the snails. 6. After irradiation of a snail's body with 64,400 R, the head being shielded, the radula remains without traces of this treatment (comp. Fig. 5, whole-body X-irradiation with 64,400 R). 7. After irradiation with low R-dosis an insignificant defect-streak develops across the radula. This was used as time-mark in order to count the cross-rows which have grown until preparation of the radula or setting of a second defect-mark. Hereby an average rate of replacement per day can be established, which may be looked upon as normal. The rate of 3.1 cross-rows per day with 48 day old snails falls to 1.4 cross-rows per day with 1 to 2.5 year old animals (Fig. 7, drawn-out graph). The average normal tooth-length increases with the age (Fig. 7, broken-lined graph). 8. To examine the effect of rising R-dosis on the rate of replacement, the cross-rows are counted which developed within 12 days after exposure (dosis: 8,050-80,500 R, Table 2 a-e). The number of cross-rows is reduced with rising dosis (Fig. 8, drawn-out graph). 9. After irradiation with 96,600 and 113,000 R groups of young snails were killed in periods of 12-168 hours after exposure (Fig. 9) to determine the actual daily replacementrate of cross-rows resulting to a high dosis. In the first 24 hours after 113,000 R the radula replacement is strongly reduced (Fig. 9, drawn-out graph). From 24-168 hours about 2.0 new cross-rows developed daily, compared to a normal rate of 2.9 cross-rows daily in animals of the same age. The pattern of malformation in the radula after a high dosis develops with slackened but very continuous replacement-rate of rows (comp. Fig. 9, broken-lined graph for 96,600 R). 10. In the epithelium at the back of the radula gland one finds several very different damages. The mitotically very active proliferation zones directly anterior and posterior to the odontoblasts-zone are considerably damaged. From 96 hours after exposure, there can be found lots of pycnotic nuclei (Fig. 18b). Extensive gaps appear in the tissue. On the other hand the odontoblasts-zone has a great compatibility against X-rays. Even 168 hours after exposure the odontoblasts are found to be completely existant. They were able to develop 12-15 cross-rows after irradiation. Merely the appearence of the odontoblasts changes. The front cells of the odontoblasts-group are lower, the posterior ones slimmer than normal. The number of radula glands with changed odontoblasts increases up to 96 hours after exposure, and decreases again until 168 hours after exposure. 11. The irregular slim rear odontoblasts have a narrowed apical area for tooth-formation (comp. Nr. 3 of the summary). Thus the tooth produced by these cells is shorter than normal. 12. The hypothesis of odontoblast-substitution states that the cells which develop the radula are continuously replaced by cells of the end-epithelium lying behind the odontoblasts-zone. After irradiation with 113,000 R replacement can not have taken place, as the tissue directly anterior and posterior to the odontoblasts-zone was extremely damaged. The odontoblasts were permanently active during the time of experiment, 168 hours after exposure. Finally they could again produce regular teeth. These findings supportthe hypothesis of the permanenec of odontoblasts.