352 MOLLUSC A. 



mates them to the Cephalophora. It is, however, impossible to say 

 that they are more nearly allied to one than to the other. 



There are three genera : Dentalium L. ; Siphonodentalium Sars ; Pulsellum 

 Stoliczka. 



In the preceding account the thin end of the animal has been spoken of as 

 posterior ; it is, however, possible to regard it as dorsal and as corresponding 

 roughly to the visceral sac of a Gastropod. 



Class III. SOLENOGA&TRES* (APLACOPHORA). 



Symmetrical vermiform animals without mantle-fold, distinct foot, 

 or shell. The integument is provided with a cuticle and calcareous 

 spicules or scales. 



It appears that the Class Amphineura, which is established by many authors 

 to include the Solenogastres and Chitonidcte is quite unjustifiable ; for, whereas 

 the Chitons are clearly Gastropods, it is by no means certain that the Soleno- 

 gastres are really Mollusca at all. Certainly they are not Gastropoda, for 

 they differ from that class in numerous features, of which we may call special 

 attention to the fact, of great morphological importance, that in them the 

 gonad opens directly into the pericardium, a feature found in no Gastropod. 



The Solenogastres comprise two families, the Neomeniidae and 

 the Chaetodermidae. They are elongated, vermiform animals with 

 a skin stiffened by the cuticle and spicules; and although in 

 the Neomeniidae there is a ventral ciliated furrow, which is sup- 

 posed to be homologous with the Molluscan foot, it may generally 

 be said of them that they are without a foot. Further, there is 

 neither mantle-fold nor shell; the respiratory structures, when present, 

 cannot be certainly homologised with ctenidia; the alimentary canal 

 is perfectly straight, passing between the anterior mouth and the 

 posterior anus; the blood is red; and finally, the head is but ill 

 marked off from the body, and entirely unprovided with special 

 organs of sense. There is a haemocOelic body-cavity, a coelom 

 consisting of a pericardium, gonad, and two nephridia; the gonad 

 communicates with the pericardium. The reasons for regarding 

 them as allied to the Gastropoda are to be found in the presence 

 on the floor of the mouth of a chitinous structure, more or less 

 closely resembling in its relations and appearance the radula; in 

 the arrangement of the central nervous system; in the fact that 



* A. A. W. Hubrecht, " Proneomenia Sluiteri" Niederl. Arch. f. Zoologie, 

 Sup^. Bd. 2, 1881. Kowalewsky and Marion, "Contributions a 1'histoire des 

 Solenogastres," Ann. Mus. Marseille (Zoologie}, 3, 1889. Pruvot, "Sur 1' organi- 

 sation de quelques Neomeniens des cotes de France," Arch. Zool. Exp. (2), 9, 

 1891. Wiren, "Studien iib. die Solenogastres," I. and II., Svenska Vet. Akad. 

 Handl, 24 and 25, 1892-3. 



