REPORT ON THE PTEROPODA. 5 



the former possess a shell at the beginning of their larval life), the groups that they 

 represent are quite natural ones. 



The following table shows the principal differences between the adult Gymnosomata 

 and Thecosomata (the two groups also present considerable differences in their embryonic 

 development) : — 



Among the other differences which exist between the two groups, the following may 

 be noticed. 



1 . The foot in the Gymnosomatous Pteropods is quite distinct from the fins ; it is 

 formed of a posterior lobe and two antero-lateral ones, joined in front, in the form of 

 two longitudinal lips. Between these two lobes a small tubercle is generally found 

 behind, formed by folds of the skin. The foot and fins in the Thecosomatous Pteropods 

 form a single and continuous mass. 



2. The penis of the Gymnosomata is latero-ventral and situated on the right side of 

 the foot. The penis of the Thecosomata is anterior and cephalo-dorsal. 



3. In the Gymnosomata there is an evaginable proboscis (the anterior protrusible 

 portion of the digestive tract) of the acrembolic type, 2 generally bearing buccal append- 

 ages, which may be completely protruded. Tn the Thecosomata there is no proboscis 

 at all. 



4. Besides the jaw and the radula, the buccal cavity of the Gymnosomata contains 

 hook-sacs, 3 a pair of evaginable sacs opening at each side of the radula. No Thecoso- 

 matous Pteropod is provided with such hook-sacs. 



5. The jaw of the Gymnosomata, composed of a great number of horny plates 

 or spines, forms only one piece, in which two halves are, indeed, visible, but these 



1 Paul Pelseneer, The Cephalic Appendages of the' Gymnosomatous Pteropoda, Quart. Joum. Micr. Sci., 1885, 

 p. 505. 



2 Ray-Lankester, Mollusca, Encyclopaedia Britannica, ed. 9, vol. xvi. p. 653. 



3 Coecums buccaux, van Beneden ; spinose tentacular organs, Huxley ; Hakensiicken, Gegenbaur ; cheek pouches, 

 buccal pouches, MacDonald ; cylindres retractiles, Fischer; hook-bearing processes of the mouth, Ray Lankester. 



