CHAPTER XV 



CLASS GASTEROPODA (^continued): OPISTHOBRANCHIATA AND 



PULMONATA 



Order III. Opisthobranchiata 



Visceral loop not twisted (except in Actaeon') in a figure of 

 8 (Euthyneurous type, p. 203), auricle usually behind the ven- 

 tricle, ctenidium often replaced by secondary branchiae, pallial 

 cavity, if existing, more or less open, shell present or absent, 

 operculum absent (except in Actaeon'), animal hermaphrodite, 

 with separate sexual openings, marine only. — Carboniferous to 

 present time. 



The character of their nervous system decisively removes the 

 Opisthobranchiata from the Prosobranchiata, and approximates 

 them to the Pulmonata. Actaeon, however, which is strepto- 

 neurous, as well as possessing an operculate shell with prominent 

 spire, forms an interesting link with the Prosobranchiata. At 

 the opposite extreme to Actaeon stand forms like Siphonaria 

 and G-adinia, which are probably close links w^ith the Pulmonata 

 (p. 19). The generative sj^stem of the whole group, which is, 

 as in the Basommatophora, of the hermaphrodite tj^pe, without 

 mutual fecundation, is another link of connexion with the 

 Pulmonata. The respiratory organs present the most varied 

 forms, sometimes consisting of one ctenidium (never two), some- 

 times of secondary branchiae, variously placed, while sometimes 

 no special organ exists. 



The prolongation of the foot into lateral epipodia or parapodia 

 (possibly to aid in swimming), and the effect of the epipodia 

 upon the shell, according as they involve it completely or par- 

 tially, are among the most instructive features of the Opistho- 

 branchiata. If the epipodia are developed on the anterior 



427 



