COMPARATIVE AX ATOMY 



CHAP. 



6 



branc/iia, 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- 

 b/'iii/chia, 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 

 Fir, 3.-Margarita Groenlandica (TrodM, after dome _ Ju mogt forms there ig Qnl 



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



3, foot ; 4, pigmented prominence at the base of the one g lU > l' laced anteriorly to the heart, 

 epipodial tentacles '(5); 6, visceral dome. 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 c 1 



wanting only in Titiscania, a genus 

 of the Xeritacea. \ 



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 

 epipoclium 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 w 7 aiiting. 



a. Zeugobranchia (Rhipidoglossa, 

 Aspidobranchia). Two gills ; both 

 auricles well developed. Heart tra- 

 versed by the rectum. Shell with 



\ 



FIG. 4. Patella vulgata (from beneath, after 



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

 tion or with a row of perforations. 

 Generally without operculum. 

 Marine. Fam. Haliotidce, radula 

 col. (5.1. 5)1 , Fissurellidce (Fissu- 

 rella, rail. eel. (4.1. 4)1. oo, with secondarily symmetrical shell. 



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

 x-y, median line; g, afferent branchial vessels 

 /, branchial lamellfe ; h, one of the afferent vessels 

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



Emurginula, Scutum 



