vii MOLLUBCA—INTEGUMENT, MANTLE, VISCERAL DOME 43 



1. Prosobranehia. 



In the Prosobranehia, the mantle fold develops on the anterior 

 side of the visceral dome, and there covers a spacious cavity. It 

 further usually extends like a narrow collar right round the base of 

 the visceral dome. 



Ill the symmetrical FissurcIlidcK, the mantle cavity is short, and opens out- 

 wardly by means of a dorsal aperture through the mantle fold, which corresponds 

 with the perforation at the apex of the shell. A circular fold, provided with a 

 liighly sensitive fringe, is formed by the mantle around the ai)erture, and projects for 

 a short distance beyond the perforation in the shell. The water needed for respira- 

 tion passes into the pallial cavity through the slit-like aperture at the free edge of 

 the mantle fold, over the nuchal region, and flows out through the apical aperture 

 just described. This aperture also serves for the ejection of excretory matter from 

 the rectum, which lies immediately behind it. In Eiinula, the apical apertures in 

 shell and mantle have moved somewhat forward, and lie anteriorly between the 

 apex and edge of the shell. In Emarginnla , the mantle has an anterior cleft, the 

 edges of which, in the living animal, are folded in such a way as to form a tubular 

 siphon, which can be protruded through the marginal cleft of the shell. In Par- 

 mophurus there is no second opening into the mantle cavity, but the lateral edges 

 of the mantle are very much widened, and bent back dorsally over the outer sui'face 

 of the shell in such a way as to cover the greater part of it. 



In Haliotis, the enormous development of the columellar muscle on the right 

 side confines the mantle cavity to the left. The mantle fold has a long slit reach- 

 ing from its edge to the base of the pallial cavity. This slit lies under a row of 

 perforations in the shell which are characteristic oi Haliotis, and through these the 

 respiratory water is expelled. In the spaces between the consecutive perforations, the 

 edges of the mantle cleft are apposed, merely separating beneath each aperture to 

 allow of free communication between the cavity and the exterior. The edges carry 

 three tentacular processes, which can be thrust outward through the perforations. 

 Tlie anus is always found under the posterior perforation. The edge of the mantle 

 surrounding the body splits into two narrow lamellre, which bend round to form 

 a groove for the reception of the edge of the shell. 



The Trorhidcc, Turhinidce, Neritidae, and nearly all Monotocardia have no second 

 aperture and no mantle cleft. 



In Docoglossa {Patella, etc.) the mantle forms a circular fold round the visceral 

 dome, which is in the form of a blunt cone. It covers the edge of the almost 

 circular broad-soled foot. The mantle is broadest anteriorly, where it covers the 

 head and neck, i.e. the pallial cavity or gi'oove is here deepest. 



The visceral dome, in the Monotocardia, is almost always distinctly constricted 

 at the base, and spirally coiled. The pallial cavity occupies its typical position. 

 In many ilonotvcurdia, the free edge of the mantle fold is prolonged on the left side, 

 projecting forward, sometimes to a great extent ; the lower edges of this projecting 

 fold bend round towards each other to form a tube or semi-cylindrical channel, 

 which is called the siphon. Through this sij)hon, the water needed for respiration 

 flows into the mantle cavity. It can generally be told, by the shape of the shell, if 

 there is a siphon or not, since most Munotucardia which possess one have either a 

 notch in the edge of the shell at the columella, or a process called the canal or beak, 

 at this same point, which encloses the siphon. The length of this latter canal need 

 not, however, correspond with that of the siphon. 



