62 INTRODUCTION. 



of conformation even in the species most remote 

 from each other ; and can easily trace the grada- 

 tions of the same plan from man to the lowest of the 

 fish. 



In the conformation peculiar to the second grand 

 division of living beings, we find no skeleton. The 

 muscles are simply attached to the skin which forms 

 a soft and contractile covering, from which proceeds, 

 in several of the species, a scaly or laminous substance 

 called shells, the position and production of which 

 are analagous to those of the mucous body. Within 

 this general envelope are the viscera and nervous 

 system, which last is composed of many scattered 

 masses, attached together by nervous threads. The 

 chief of these masses placed in the oesophagus re<- 

 ceives the denomination of the brain. Of the senses, 

 properly so called, we can seldom distinguish, 

 among these animals, more than the organs for 

 those of taste and vision, and we sometimes find 

 that even these are wanting. One family alone ex- 

 hibits the organs of hearing. In other respects this 

 division is characterized by a complete circulating 

 system, and peculiar organs of respiration. The 

 apparatus for digestion and secretion are scarcely 

 less complicated than are those of the vertebrata. 



We give to the animals whose conformation is 

 modelled according to this second form, the appella- 

 tion of Mollusca. 



Although the general plan of their organization is 

 not so uniform as that of the vertebrata in relation 



