COMPARATIVE AXATOMY 



CHAP. 



Fig. 3.— Margarita Groenlaudlca (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. 



hranchia, 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- 

 hmnchiu, 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 

 dome. In most forms there is only 

 one gill, 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 apertm'e of the shell. A shell is 

 wanting only in Titiscaniu, a genus 

 of the Xi:ritari:a. 



Sub-Order 1. Diotocardia. 



The heart has two auricles (except- 

 ing in Docoglossa). There are two 

 kidneys. Instead of thepedalganglion 

 of other Gastroiioda, there are two 

 longitudinal nerves in the foot, sup- 

 plied with ganglia and connected with 

 one another by numerouscomraissures. 

 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 arc all wanting. 



a. Zeugobranchia (Rhipidoglossa, 

 Aspidobrancliia). — Two gills ; both 

 auricles well developed. Heart tra- 

 versed by the rectum. Shell with 

 marginal cleft, or with apical perfora- 

 tion or with a row of perforations. 

 Generally without operculum. 

 Marine. Fam. HaliotiJa;, radula 

 c^jI. (5.1.5)1=0, Fissurcflldii: {Fissu- 

 rella, rad. ccl.(4.1,4)l.«) , with secondarily symmetrical shell. Eincyimda, .S'aituvi 



Fig. 4.— Patella vulgata (from beneath, after 

 Lankester). «, Tentacle ; d, eflerent branchial vessel ; 

 c, free edge of the shell ; e, free edge of the mantle ; 

 J--!/, median line; </, afferent branchial ves.sels ; 

 /, branchial laniellie ; li, one of the afferent vessels '; 

 i, spaces between the shell muscles ; h, foot. 



