394 GASTEROPODA. 



condition, and complete their growth in a situation where, being 

 diffused over a larger surface, the shells may be closely approxi- 

 mated ; and, moreover, the eggs and their contained offspring are 

 by this contrivance freely exposed to the influence of the medium 

 around, so as to allow a kind of respiration to be enjoyed by the 

 unhatched young. The situation chosen is the branchial fringes, 

 over which the imperfect spawn, or spat, as it is technically 

 termed, is found widely spread towards the close of gestation, 

 still retained beneath the shelter of the shell of the parent, and 

 thus preserved from destruction ; but at the same time, being in 

 such a position freely washed by the ciliary currents, the respiration 

 of the included embryo is adequately provided for. 



CHAPTER XXIII. 



GASTEROPODA.* (Cuv.) 



(429.) EXTENSIVELY distributed over the surface of the land, or 

 inhabiting the waters either fresh or salt, there exists a very nu- 

 merous body of Mollusca, differing widely among themselves in 

 construction and habits, but distinguished by a peculiar locomotive 

 apparatus common to the entire class, by means of which they are 

 able to fix themselves to plane surfaces, and to move from place to 

 place by a slow and gliding motion. The slug, the snail, the 

 limpet, and the welk, afford familiar examples of their general 

 form and external appearance ; but species of different kinds are 

 so common in every situation, that it would be wasting the time 

 of the reader to dwell at any considerable length upon their ordi- 

 nary configuration and usual mode of progression. 



The bodies of the GASTEROPODA are frequently entirely soft, 

 and devoid of other covering than a thick and slimy skin ; but 

 more generally they are protected by a shell of very diverse form 

 and shape, into which they can retire for protection. Feeble and 

 languid as are the sluggish movements of these creatures, they 

 nevertheless present to the eye of the anatomist a type of or- 

 ganization considerably superior to any that we have had an oppor- 

 tunity of considering in such forms of the HETEROGANGLIATA as 

 have been described in the preceding chapters. From the supe- 

 riority of their mode of progression, it is evident that they are 

 adapted to enjoy a less limited intercourse with external objects 

 than even the most highly gifted of the burrowing CONCHIFERA ; 



* yaffT*i, the belly ; wawj, a foot. 



