CHAPTER XV 



CLASS GASTERoroDA {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 ventricle, 

 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, wdiich is strepto- 

 neurous, as well as possessing an opcrculate shell with prominent 

 spire, forms an interesting link with the Prosobranchiata. At 

 the opposite extreme to Actaeon stand forms like Sijyhonaria 

 and Gadinia, which are probably close links with the Pulmonata 

 (p. 19). The generative system of the whole group, which is, 

 as in the Basommatophora, of the hermaphrodite type, 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 

 partially, are among the most instructive features of the 

 Opisthobranchiata. If the epipodia are developed on the 



