vii MOLLUSCA INTEGUMENT, MANTLE, VISCERAL DOME 



The mantle 



aperture is left, through which the dorsal surface of the shell is visible. 

 in Dolalclln forms a small anal siphon posteriorly. 



Notarchus has a microscopically minute shell. In certain species of this genus, 

 the integument forms protuberances or delicately 

 branched appendages. A 



In the Oxynoidcit, the shell is only partially covered 

 by the mantle, and is, further, much too small to 

 shelter the body. 



Among the Notaspiihc, the UinbreUidec have a 

 small flattened cap-like visceral dome lying upon the 

 massive foot. The visceral dome is surrounded by a 

 mantle fold which, on the right side, covers the gill. 

 The integument of the dome and mantle is covered 

 by a flattened disc-shaped shell. 



In Pleurobraiii'/iii/, the visceral dome is relatively 

 large. The right and left margins project as short 

 mantle folds, but there are no such folds to the front 

 and back, so that at these latter parts the flattened 

 visceral dome is not distinct from the rest of the body. 

 In Plfurolj-ranehus, the integument of the flatten*-. 1 

 visceral dome broadens out into a large fleshy disc 

 which projects on all sides beyond the large, broad - 

 soled foot ; its margin (mantle fold) is separated from 

 the foot by a deep continuous groove running right 

 round the body ; in this groove, to the right, lies the 

 large gill, while in Pleurobranchus a small flat internal 

 shell, thin and membranous, is still found ; in related 

 forms this may be wanting. The dorsal integu- 



layer of calcareous 



ment is often strengthened by a 

 granules. 



FIG. 56. Diagrammatic trans- 

 verse sections of Gastropods, to 

 illustrate the arrangement of the 

 shell (black, 1), visceral dome and 

 mantle (dotted, 2), and foot (streak- 

 ed, 3). A, Prosobranchiate with 



in -.-I , i , , outer shell and epipodium (4). B. 



shell, with simstral twist ; the shell can be closed by Abranchiate with lobes (6) of 



means of a typical operculum. The mantle fold covers the mantle turned back over the 

 a cavity which lies anteriorly to the visceral dome, outer surface of the shell. Dorsally 



Q3) Natantia. 



Pteropoda Thecosomata. The Limaciiiittcc have 

 a well - developed visceral dome and corresponding 



The anus is to the 

 draw into its shell. 



right. The animal can with- 

 in the Cavoliniidcc the dome 



the shell is still uncovered ; 5, para- 

 podia ; 7, ctenidium. C, Tecti- 

 branchiate with internal shell, i.e. 

 and shell are bilaterally symmetrical, not twisted, and comple tely overgrown by the lobes 

 the body can be entirely hidden within the shell, of the mantle. 

 The mantle cavity here lies posteriorly to the 



visceral dome, on what is usually called its lower side. The symmetrical shell of 

 the CymbuliidK does not correspond with the shell of other Thecosomata ; it is a 

 cartilaginous " pseudoconch " covered with body epithelium. In the Cymbal ii<l,i 

 the mantle cavity also lies posteriorly. We shall return later to the varying 

 position of this cavity among the Thecosomata. 



The mantle, in the genus Cavulinin, shows peculiarities which can best be described 

 in connection with the shell. In the latter, two surfaces are distinguished, a slightly 

 arched anterior surface (usually described as the upper), and an arched posterior 

 surface. The anterior surface projects forwards and downwards beyond the 

 posterior for a third of its length. The shell has three slit-like apertures, one 



