236 SYNOPTIC COLLECTION. 



its long narrow foot with thin and flexible sides is well 

 adapted for clasping the stems of algae. 



The branchiae in Pleurophyllidia (No. 527, P. semperi 

 Bergh), consist of leaf-like organs on each side of the 

 body between the foot and the dorsal portion of the 

 mantle. In Doris (No. 528, D. nubilosa Pease) they 

 form a circle around the anus and are retractile, while in 

 Trevelyana (No. 529, T. cristata Bergh) and Plocamo- 

 phorus (No. 530, P. imperialis Angas) they are not so 

 completely retractile, although according to Eliot, 1 they 

 retract when touched in the living animal, but remain out- 

 side in alcoholic specimens. 



Among the Nudibranchs that have lost both shell and 

 branchiae are Phyllobranchus (No. 531, P. orientalis 

 Kol.) and Elysia (No. 532, E. chlorotica Gould) ; while 

 in Pontolimax (= Limapontia) (No. 533, P. capitatus O. 

 F. Mull.) no shell, branchiae, nor appendages exist. 



It is interesting to note that the law of acceleration in 

 development has brought about a condensation of embry- 

 onic stages in two Nudibranchs, Cenia cocksi and Pelta 

 coronata? so that the veliger stage with the ciliated velum 

 and the shell is omitted from the ontogeny. 



SCAPHOPODA. 



The Scaphopoda are a small group represented by Den- 

 talium (Nos. 534-536). The tube like shell of this genus 

 is not made by the addition of successive layers to the 

 margin of a cone-like embryonic shell, as a primitive 

 Gastropod shell would be formed. On the contrary, the 

 mantle at first consists of two folds which in the process 

 of development come together on the ventral side and 



1 Proc. Acad. Nat. Sci. Phila., 1899, P- 5 2 - 



2 Zool. Anz., XXIII, 1900; quoted in Science, n. s., XII, 1900, 

 p. 824. 



