THE BRANCHIOGASTEROPODA. 437 



group, the development of which has hitherto been studied, 

 the intestine becomes twisted round on to the anterior face 

 of the body, in such a manner that the alimentary canal has 

 a completely haemal flexure, even in the veligerous embryo. 

 Hence, in the adult, the intestine springs from the haemal or 

 dorsal, and not from the ventral or neural, aspect of the 

 stomach ; and the pallial cavity, when it exists, is placed 

 upon the anterior haemal face of the body. 



In the embryo, the shell always makes its appearance as 

 a conical, symmetrical, median cap. This embryonic shell 

 usually persists at the apex of that of the adult, the form of 

 which is modeled upon that of the visceral sac, and hence, 

 like the latter, is usually spiral. The embryo is also very 

 generally, if not universally, provided with an operculum. 



The shell and operculum of the embryo disappear in the 

 naked Branchiogasteropods ; but the primitive external shell 

 is sometimes replaced by an internal shell lodged in a cavity 

 of the mantle (e. g., Aplysia). Usually, the Branchiogastero- 

 pods possess a distinct head provided with a pair of tentacles 

 and with two eyes, which may either be sessile or mounted 

 upon peduncles of their own. 



The mouth may be armed with chitinous jaw-plates, in ad- 

 dition to the radula. The heart is generally composed of a 

 ventricle and a single auricle, but sometimes there are two 

 auricles. 



The Br anchio gasteropoda fall into two distinct series, of 

 which the one is hermaphrodite (the genital gland being an 

 ovotestis) and invariably opisthobranchiate ; while the other 

 is unisexual and usually prosobranchiate. In each series 

 there are some forms which are provided with a large mantle, 

 and others in w 7 hich the mantle is altogether abortive (Nudi- 

 branchiata, Fir old). These chlamydate and achlamydate 

 Branchiogasteropods correspond with the Thecosomata and 

 Gymnosomata among the Pteropods. 



The chlamydate Branchiogasteropods are usually provided 

 with branchiae, which either take the form of numerous la- 

 mellae, or of two plume-like organs, sometimes reduced to one 

 functional gill and a rudiment of the second. In the achlamy- 

 date forms true gills are usually absent, though they may be 

 replaced functionally by processes of the haemal body-w r all. 



Among the Opisthobranchiata, Phyllidia is nearly sym- 

 metrical, the anus being situated at the posterior end of the 

 body, and there is a large mantle, devoid of a shell. There 



