vii MOLLUSGA INTEGUMENT, MANTLE, VISCERAL DOME 45 



near it in a way characteristic of the Pulmonata, leaving only one aperture, the 

 respiratory aperture on the right. (In Pulmonata whose shells have the sinistral 

 twist, the respiratory aperture lies to the left. ) The apertures of the hind-gut and 

 excretory organ are close to the respiratory aperture, through which their excreta 

 have to pass out. 



In many species of the genus Vitrina, the shell cannot contain the whole animal. 

 The mantle fold projects in front of the shell, and has a process which is bent back 

 over the shell, and is used for cleansing it. 



In Daudebardia (Helicophanta} (Fig. 12 B, p. 9) the visceral dome and shell are, 

 in comparison with the rest of the body, much smaller than in Vitrina. The animal 

 cannot be sheltered by the shell. The visceral dome begins to be levelled down to a 

 certain extent, disappearing into the dorsal surface of the foot. It lies far back 

 on the body, the respiratory aperture being on its right side. 



A somewhat similar arrangement is found in the genus Homalonyx, in which the 

 low visceral dome lies on 

 the centre of the back. 

 The respiratory aperture 

 lies to the right at the edge 

 of the mantle. The edge 

 of the flat ear-shaped shell 

 is fixed into the mantle 

 fold. Daudebardia and 

 Homalonyx begin to look 

 like slugs. 



In Testacella (Figs. 54 

 and 55) a visceral dome 

 hardly exists. The only re- 

 mains of it is a small mantle 



at the dorso-posterior end FlG 5 5 ._Testacella haliotidea, posterior portion of the body 

 of the body, which is from the right (after Lacaze-Duthiers). The shell is removed to 

 covered by an ear - shaped show the rudimentary visceral dome, a, latero-dorsal groove ; 6, 

 shell Beneath the mantle l a tero-ventral groove ; c, end of the muscle attached to the shell ; 

 , . e. mantle edge of the visceral dome ; g, respiratory aperture, 



lies a reduced respiratory 



cavity. The respiratory aperture lies to the right posteriorly, beneath the edge of 

 the shell. The viscera lie dorsally on the foot. 



The common terrestrial snails Limax and Arion (Fig. 12 D, p. 9) resemble 

 Testacella in the reduction of the visceral dome, but in them the mantle or so-called 

 shield which takes its place lies anteriorly behind the head. At its right edge lies 

 the respiratory aperture. In Limax there is a small round rudimentary shell which 

 is internal, i.e. it is entirely enveloped in or overgrown by the mantle [fold. In 

 Arion this shell is represented by isolated calcareous granules. In Onchidium and 

 Vaginulus there is no trace of a visceral dome, nor, in the adult, of a shell. The 

 visceral dome has to a certain extent spread out over the whole dorsal surface of 

 the foot, and has disappeared. There is, further, no outwardly recognisable mantle 

 fold distinct from the rest of the dorsal integument. A longitudinal furrow still 

 divides the dorsal part of the body from the foot. The respiratory aperture with 

 the anus lie posteriorly in the median line. 



In the genus Physa (Fig. 11, p. 8), the edge of the mantle takes the form of 

 lobe-like or finger-shaped processes, which bend back over the shell, and can be 

 applied to its outer surface. In Amphipeplea (Fig. 10, p. 8) the mantle is much 

 widened and, when bent back over the shell, covers all but an oval spot on the 

 dorsal side of the last coil. 



The dorsal integument of the Onchidia has wart-like protuberances or (in Peronia] 



