318 Dr. T. Williams on the Mechanism of Aquatic 



epithelium. This is actually the case in the freshwater Limneids, 

 the most highly ciliated of this order. In no single species is 

 there discoverable a space of any description, or a layer of chan- 

 nels or vessels of any sort between it and the invested abdominal 

 organs. It is therefore certain that the abdominal segment of 

 the body in the conchiferous Gasteropods can lend no aid what- 

 ever in the function of breathing. The porcellaneous interior 

 surface of the shell is totally devoid of all organic covering ; its 

 polished surface is well adapted for the ready motion of the soft 

 abdomen, covered also by its finely polished membrane, in its 

 coiled chamber. The nacreous internal layers of the shell answer 

 another important end : they render the shell water-proof; 

 that is, the water which is drawn into the abdominal cavity, 

 through the space at the mouth of the shell between the thoracic 

 chamber and the edge of the latter, cannot escape by exosmosis 

 or evaporation through the texture of the shell. The aperture 

 of the shell being closed, by the operculum and thoracic expan- 

 sion of the mantle, the water in the spire cannot escape. It 

 constitutes a permanent reservoir. In this position it answers 

 important purposes. It prevents the drying of the delicate ab- 

 dominal membrane. It preserves the vital organs underneath in 

 the required condition of moistness. It obviates the exhaustion 

 of the nutritive fluids by evaporation. But more than all, it 

 is capable, by a slight movement backwards of the animal, of 

 being pushed forward into the cavity of the anterior shell-coil, 

 and thence it may be drawn into the branchial chamber. In 

 the littoral families of Univalves this is not a useless function 

 when they are left on the dry rock by the recession of the tide. 

 In the land Helicidse it subserves also a similar end. This fluid 

 in the Cyclobranchiata is lodged in the hollow of the apex or 

 vault of the shell (fig. 1 o) resting on the back of the animal. 

 In this order the border of the mantle is deficient at the point 

 {b)y which corresponds with the back of the neck of the animal. 

 It is by this orifice or passage that the water is drawn into the 

 cavity [a). The character, the extreme dilatability and uses of 

 this cavity may be advantageously studied in the common 

 Patella. It is capable under certain circumstances of receiving 

 and retaining a large volume of fluid. But the furthest limits 

 (c) to which it may be distended without rupture are best shown 

 by the injection of coloured size. By steadily continued force, 

 the fluid thrown in will slowly thrust the mollusk out of its 

 shell [i). It forms a fulcral point upon which the animal bears, in 

 the act of fixing its shell to the rock. This dorsal chamber in 

 the Chitonidse is very small. In the Fissurellidaj the perforation 

 at the apex of the shell opens into the cavity ; the latter cannot 

 therefore act on the mechanical principle above explained. But 



