vii MOLLUSC A INTEGUMENT, MANTLE, VISCERAL DOME 47 



The mantle 



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

 in Dolabella 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 Oxynoidea, the shell is only partially covered 

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

 shelter the body. 



Among the Notatpidce, the Umbrellidee 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 Plcurobranchia, 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 Pleurobranchus, the integument of the flattened 

 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 Pleiirobranchus a small flat internal 

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

 forms this may be wanting. The dorsal integu- 

 ment is often strengthened by a layer of calcareous 

 granules. 



(/3) Natantia. 



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 

 outer shell and epipodium (4). B, 

 Tectibranchiate with lobes (6) of 

 the mantle turned back over the 

 outer surface of the shell. Dorsally 

 the shell is still uncovered ; 5, para- 

 podia ; 7, ctenidium. C, Tecti- 

 branchiate with internal shell, i.e. 

 completely overgrown by the lobes 

 of the mantle. 



Pteropoda Thecosomata. The Limadnidce have 

 a well - developed visceral dome and corresponding 

 shell, with sinistral twist ; the shell can be closed by 

 means of a typical operculum. The mantle fold covers 

 a cavity which lies anteriorly to the visceral dome. 

 The anus is to the right. The animal can with- 

 draw into its shell. In the Cavoliniidce the dome 

 and shell are bilaterally symmetrical, not twisted, and 

 the body can be entirely hidden within the shell. 

 The mantle cavity here lies posteriorly to the 



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

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

 cartilaginous " pseudoconch " covered with body epithelium. In the Cyinbuliidoe 

 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 Cavolinia, 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 



