COMPARATIVE ANATOMY 



CHAP. 



6 



FIG. 3.-Margarita Groenlandica (Trochid, after 

 Pelseneer). 1, Head ; 2, anterior epipodial lobes ; 

 3, foot ; 4, pigmented prominence at the base of the 

 epipodial tentacles (5) ; 6, visceral dome. 



branchia, this shell may become more or less rudimentary (generally in connection 

 with the reduction of the visceral dome). 



The pallial complex becomes shifted forward on to the right (seldom the left) 



side, or along this side so as to lie quite 

 anteriorly. The visceral dome and 

 shell (with some exceptions) are spirally 

 coiled. 



In all except the lowest Proso- 

 brancMa, the asymmetry is evidenced 

 by the disappearance of one gill, of one 

 kidney, and of one auricle. 



The radula is rarely wanting. 



ORDER 1. Prosobranchia. 



The pleuro- visceral connectives are 

 crossed. The mantle complex is twisted 



round to the front side of the visceral 

 domei Jn mogt formg there ig on] 



one gA placed anteriorly to the heart, 

 and in the heart the auricle lies anter- 

 iorly to the ventricle. The Proso- 



branchia are chiefly marine, and are sexually separate. The foot is generally pro- 



vided with an operculum for closing 



the aperture of the shell. A shell is 



wanting only in Titiscania, a genus 



of the Neritacea. 



Sub-Order 1. Diotocardia. 



The heart has two auricles (except- 

 ing in Docoglossa). There are two 

 kidneys. Instead of the pedal ganglion 

 of other Gastropoda, there are two 

 longitudinal nerves in the foot, sup- 

 plied with ganglia and connected with 

 one another by numerous commissures. 

 The gills are feathered on two sides, 

 their points projecting freely. The 

 epipodium is well developed, and 

 there is a circle of more or less 

 numerous tentacles around the base 

 of the foot. Proboscis, penis, and 

 siphon are all wanting. 



a. Zeugobranchia (Rhipidoglossa, 

 Aspidobranchia). Two gills ; both 

 auricles well developed. Heart tra- 

 versed by the rectum. Shell with FlG . 4 ._p a tella vulgata (from beneath, after 

 marginal cleft, or with apical perfora- Lankester). , Tentacle ; d, efferent branchial vessel 

 tion or with a row of perforations. c > free ed s e of the she11 ; e > free ed s e of the mantle 

 Generally without operculum. *-*> "^ "*' ?, afferent branchial reweta 

 * * i i /i branchial lamellae; h, one of the afferent vessels 



Marine. Fam. ffahotidce, radula i? spaces be tween the shell muscles ; b, foot. 

 ool.(5.1.5)loo, Fissurellidce (Fissu- 

 rella, rad. ool.(4.1.4)l.oo, with secondarily symmetrical shell. Emarginula, Scutum 



