180 



INTRODUCTION TO ZOOLOGY. 



see the Lynmaea raid Plauorbes (Figs. 147, 168) thus 



r\ 



Fig. 1GS. PLANORIIIS. 



traversing the surface of ponds and ditches, in an easy undu- 

 lating line, or suspended there in luxurious repose, perhaps 



" To taste the freshness of heaven's breath, and feel 

 That light is pleasant, and the sunbeam warm." 



The soft skin of those species which are unprotected with 

 shells might naturally be supposed to be possessed of great 

 sensibility, but such does not appear to be the case. " Baron 

 Ferussac, for example, states that he has seen the terrestrial 

 Gasteropods or slugs allow their skins to be eaten by others, 

 and, in spite of large wounds thus produced, show no sign of 

 pain."* They possess, in a high degree, the power of repair- 

 ing injuries and of reproducing lost parts. Many species, in 

 their young state, can suspend themselves from any object by 

 means of a thread emitted for the purpose, and in some this 

 thread-producing power continues during life.f Those who 

 have not examined the internal structure of these animals may 

 perhaps be surprised to learn that in each there exists a small 

 rudimental shell. If we are asked "what is the use of it?" 

 w r e can only answer, "we cannot tell;" but, in many other 

 animals, we can point to a rudimental structure apparently of 

 no use in the organization of a certain species, yet, in others 

 with which it is nearly allied, becoming, in its full development, 

 of great importance to the economy and habits of the animal. 



Thus, in the present case, though we find only a rudimental 

 shell in the Slug (Limax\ we meet with a conspicuous ex- 

 ternal covering of shell in the Snail (Helix). The species 

 belonging to the latter family (Helicidce) are very numerous, 



* Quoted by Owen, page 30G. 



f Rev. B. J. Clarke, on the Irish species of the Genus Limax. Annals 

 Nat. Hist, vol. xii. page 341. 



