ir. GASTEROPODA : OPISTIIOBRANCIIIA. 



381 



Order II. Opisthobranchia. 



The Opisthobran cilia liave not varied from the primitive s5-m- 

 metry to such an extent as liave Prosoljrauchs and Pulmonates. 

 The anus is in the plane of symmetry or only slightly removed 

 from it, although it may be placed far forwards. Tlie nervous 

 system is orthoneurous, the twist being straightened (except in 

 Actaeonidse). The heart also retains its primitive 2>osition, receiv- 

 ing blood from behind and forcing it forward to the body through 

 the aorta (fig. 30!)). In rare cases a (right) ctenidium, a poorly 

 developed mantle, and a thin shell enveloped in the latter occur. 

 Usually these have been lost and the place of the ctenidium is 

 taken by accessory gills of various forms or a dermal respiration 



Fig 376 —ffvitea compiniififa from above. (After Cxegenbaur.) a, arms; hr, giU ; c, 

 heart: (/, gonad: h, liver: m. mantle; oe, esophagus; )c, neiiUridium ; v. 

 stomach; //, pedal ganglion and otocyst. 



occurs. It is interesting to note that the larv!\? have well-developed 

 mantle and shell. Also important from the systematic standpoint 

 is the existence of hermaphroditism, the genital duet opening 

 on the right side. Many of the Opisthobran chs afford fine 

 examples, in form and coloration, of protective resemljlance. All 

 are marine. 



Sub Order I. TECJTIBRANCHIA. Mantle and usually a shell and 

 ctenidium jjresent, ii:irapo(li;d processes often present. Sraphuiider* 

 Biilhir PIiHiiK','"- Aphjsid. 



