316 Dr. T. Williams on the Mechanism of Aquatic 



to do more than to illustrate types and plans of structure by 

 special reference to a few familiar examples. Little account will 

 be rendered of those classes with reference to which no personal 

 opportunities have occurred to the author for instituting original 

 investigations. 



The Pteropoda are thus first excluded. 



In proceeding to the study of the respiratory system of the 

 Gasteropod mollusks, there present themselves a few questions 

 which must be preliminarily considered. 



In the conchiferous orders of this class, the shell is not simply 

 and exclusively intended to shield and protect from external 

 violence the soft body of the animal : it is a means by which the 

 animal maintains between itself and the surrounding medium a 

 certain fixed and important relation. The soft parts are tied to 

 the shell only at certain limited points (PI. IX. fig. 2*^). The 

 mode of connexion differs in different orders, but not in families 

 and genera. The point or line of attachment {b) is the pivot 

 upon which the motions of the body of the animal are performed. 

 Locomotion, in which the animal as vv'cll as the shell effect a 

 change of place, is accomplished by means of the foot. The 

 movements of the animal within the shell, the latter being fixed, 

 considered with respect to its inunediate well-being, assume a 

 far greater importance than that of progression. In all shelled 

 Gasteropods, the shell, whatever be its figure, whether univalve 

 or multivalve, spiral, tubular or conical, circumscribes a chamber 

 which is larger in size than the body of the animal which it 

 serves to lodge. The difference in sectional area in the case of 

 the spiral Univalves between the solid coil of the contained body 

 and the hollow coil (fig. 2 a, a, a, a) of the containing shell, in- 

 dicates a space which in all instances is filled with water : it is 

 water which occupies this space even in the land Helicidae. In 

 the Patelloid and Chitonoid families (fig. 1 a, Z») a chamber cor- 

 responding to this space is bounded above by the hollow apex of 

 the shell, below by the dorsal surface of the animal ; it is closed 

 behind and open anteriorly. When the occupant extrudes itself 

 from this coned chamber, a spacious open cavity is generated at 

 the apex of the shell, into which thi'ough an opening in the edge 

 of the mantle [b), situated above the head of tlie animal, water 

 or air rushes. In the Pissurellidse, in which the apex of the 

 shell is perforated, the surrounding element ia part enters into, 

 and esca])cs from, according as tlie inmate emerges from or retires 

 into its shell, the space at this orifice, the edge of the mantle 

 consequently being adherent nearly all round. This is a point 

 of generic difference between the Patellidre and Fissurellidse. 

 The layer of water intervening between the animal and the shell 

 is in direct contact with that region of the body in which the 



