IV. GASTEROPODA: OPISTHOBRANCHIA. 



381 



Order II. Opisthobranchia. 



The Opisthobranchia have not varied from the primitive sym- 

 metry to such an extent as have Prosobranchs and Pulmonates. 

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

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

 system is orthoneurous, the twist being straightened (except in 

 Actseonidae). The heart also retains its primitive position, receiv- 

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

 the aorta (fig. 369). 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. ZlG.Hyalcea complmata from above. (After Gegenbaur.) a, arms ; 6r, gill ; c, 

 heart ; </, gonad ; 7i, liver ; m, mantle ; oe, oesophagus ; re, nepliridium ; v, 

 stomach ; 7J, pedal ganglion and otocyst. 



occurs. It is interesting to note that the larvae have well-developed 

 mantle and shell. Also important from the systematic standpoint 

 is the existence of hermaphroditism, the genital duct opening 

 on the right side. Many of the Opisthobranchs afford fine 

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

 are marine. 



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

 ctenidium present, parapodial processes often present. /Scaphander,* 

 Bulla* Philine* Aplysia. 



