DEVELOPMENT OF THE EXTERNAL FORM PTEROPODA. 173 



be seen as very small, rounded lobes projecting from depressions in 

 the body (KROHN). 



The further metamorphosis consists in the growth of these parts 

 and the degeneration of the ciliated rings. The most anterior of 

 these is the first to disappear and then the middle ring ; the posterior 

 ring is still to be found when the young animal attains its full size, 

 but no doubt degenerates later. 



We must here add a few words of explanation as to the position assigned by 

 us to the Pteropoda. Until recent times, the Pteropoda were often regarded 

 as a special class equivalent to the Gastropoda, Cephalopoda, etc., although 

 some zoologists objected to such a classification. For anatomical and onto- 

 genetic reasons the Pteropoda are now classed with the Gastropoda,* being 

 placed specially near the Opisthobranchia, as is indicated by the form of 

 the central nervous system and their circulatory apparatus, as well as by the 

 structure of their genital ducts and by their hermaphroditism. Another im- 

 portant factor in classing the Pteropoda is found in the organ which gives the 

 body its characteristic shape, viz., the swimming apparatus. The manner in 

 which the fins arise proves that they are derived from the transformed lateral 

 parts of the foot. It is an interesting fact that in some Pteropoda (the 

 Gymnosomata) the propodium has still retained its function as a creeping 

 sole, serving, like the sucker of the Heteropoda, for attachment (SOULEYET, 

 No. 121 ; GROBBEN, No. 39). The fins have been regarded by some as epipodia, 

 but PELSENEER, on the contrary, considers them to be widenings of the whole 

 margin of the foot. Such fin-like widenings (swimming lobes) are found in 

 certain Opisthobranchs, and the derivation of the Pteropoda from such forms 

 seems to be suggested. GROBBEN, as well as BOAS and PELSENEER (No. 84),. 

 the two more recent investigators of this subject, have recently given active 

 adherence to this view. Lateral widenings of the sole of the foot are found in 

 Acer a, Gasfaropteron. These Opisthobranchs which, like the Pteropoda, can, 

 swim freely by flapping these fin-like foot-lobes have therefore been regarded 

 as the starting-point for the latter group. From such Opisthrobranchs the 

 Thecosomata would first have to be derived, as has been done by PELSENEER, 

 who traced back the Thecosomata to forms like Accra among the Bulloidea, 

 whereas he derives the Gymnosomata from forms like Aplysia, in which latter 

 the swimming lobes are, as in the Gymnosomata, somewhat more dorsal in 

 position. PELSENEER, in his classification of the Opisthobranchia, places the 

 Thecosomata directly after the Bulloidea, and the Gymnosomata near the 

 Aplysoidea. BOAS also regards the Pteropoda as very nearly related to the 



* Among the maintainers of this view we may mention FOL, SPENGEL,. 

 GROBBEN, BOAS, and PELSENEER. In It. HERTWIG'S text-book, the Pteropoda, 

 are classed as a subdivision of the Gastropoda, and GLAUS also recently gives 

 them a similar position, placing them after the Opisthobranchia. [Practi- 

 cally all zoologists now class the Pteropoda with the Gastropoda and most 

 accept PELSENEER'S views according to which they find their nearest allies, 

 in the Tectibranchiate Opisthobranchs. PELSENEER further separates the 

 Gymnosomata from the Thecosomata, placing the latter with the Bulloidea 

 and the former with the Aplysoidea (see Challenger Reports, Vol. xxiii.) . 

 ED.] 



