4 



GOMFA h'A riVE A XA TOMY 



Fig. 3.— Margarita Groenlandica (Trochiil, after 

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

 3, t'(jut ; 4, pij^juenteil prominence at the base of the 

 epipodial teiitacles (5) ; e, visceral dome. 



hranchia, this shell may become more or less rudimentary (generally in eoiinection 

 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- 

 bmnchld, the asymmetry is evidenced 

 liy the disappearance of one gill, of one 

 kidney, and of one auricle. 



The radula is rarely wanting. 



jl /. \ fy'K ^ 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 ojjerculura for closing 



the apertiu'e of the shell. A shell is ^^ ^ 



wanting only in Titiscaiiia, a genus ^ e 

 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 Gastroi)oda, 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 arc all w'anting. 



". Zeugobranchia (Rhipidoglossa, 

 Aspidobranchiai. — Two gills : liotli 

 auricles well developed. Heart tra- 

 versed by the rectum. Shell w'ith 

 marginal cleft, or with apical perfora- 

 tion or with a row of perforations. 

 Generally without operculum. 

 Marine. Fam. Haliotidce, radula 

 a>l.(5.1.5)loo, FissurcUidce (Fissu- 

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



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

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

 C-, free edge of the shell ; e, free edge of the mantle ; 

 X ■ y, median line ; g, afferent branchial vessels ; 

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

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



Emargin ula, Scutum 



