ECOLOGY— DEPTH DISTRIBUTION 



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Suborder Gymnosomata (=Pterota) 

 Families Thliptodontidae and Pneumodermatidae 



Table 6. Occurrences of Gymnosomata 



Thliptodon diaphanus Meisenheimer 



1905 Thliptodon diaphanus Meisenheimer, Pteropoda, Wiss. Ergebn. D. Tiefsee-Exped. 'Valdivia', ix, 59. 



1926 Thliptodon diaphanus Pruvot-Fol, Moll. Pterop. gymnos. Res. Camp. Sci. Monaco, lxx, p. 28, pi. 3, fig. 79. 



1942 Thliptodon diaphanus Pruvot-Fol, Gymnosomata I, Dana Reports, xx, 23, figs. 20-5. 



1950 Thliptodon diaphanus Tesch, Gymnosomata II, Dana Reports, xxxvi, 22, figs. 10, 11. 



Thliptodon diaphanus was the only gymnosomatous pteropod beside Pneumodermopsis paucidens to be 

 taken by the first survey, and it occurred as one specimen only at 250-500 m. at WS 996. Its recorded 

 distribution shows it to be an essentially oceanic species with a wide range over all the warmer regions. 

 In the Atlantic Ocean (Massy, 1917) it reaches the west coast of Ireland (52 N) while in the Pacific 

 it seems to attain its southernmost extreme at 45 ° S on the New Zealand coast. It is a species of the 

 upper layers as well as of the deep sea. 



The leading character of the genus Thliptodon is the enormously swollen head, which contains a 

 pair of large bladders (' Schlundblasen ') and two very roomy hook-sacs, in which lie a number of 

 golden-coloured hooks. The number of these is of taxonomic importance. Otherwise the only reliable 

 specific character is the radula. The fins form a pair of narrow outgrowths at the constriction between 

 the enlarged head and the rest of the body, and frequently — as in this specimen — both they and the 

 lobes of the foot are difficult to see, being withdrawn into folds of the baggy skin. In the specimen 

 dealt with here, the number of hooks in either sac is about fifteen, decreasing in length inwards as is 

 the case in diaphanus. 



My figure of the radula does not perfectly agree with either of those of Pruvot-Fol (1926, 1942) or 

 with that of Tesch (1950), but the differences are not hard to reconcile among these variant drawings. 

 Pruvot-Fol's (1926) figure and my accompanying one (Fig. 4a) both show a narrow arcuate trans- 

 verse bar, all that can be seen of the median tooth. Tesch (1950), however, is probably correct in 

 figuring this tooth as a broader plate which is for the most part extremely thin, fragile and transparent, 

 and impossible to see in my probably damaged specimen. The anterior edge of the tooth, however, 

 is considerably more thickened than would appear from Tesch's figure, and I was able to detect a row 

 of the finest denticulations, like those figured by Pruvot-Fol (1926). At the inner edge of the inter- 

 mediate plate is a short but strong spine. In the specimen figured, this did not appear so strongly 



