402 ' INVERTEBRATES. 



mass, without parts, and almost without organization. Their 

 muscles are fix^ into the ^kin, which is naked, very sensible, and 

 constantly moistened by a fluid furnished by its pores. The con- 

 tractions of these muscles produce certain obscure and indistinct 

 motions of their whole bodies, by means of which they are enabled 

 to swim and crawl, or even seize those objects which are adapted 

 to their nourishment. But as no part is supported by any solid 

 foundation, like the bones of vertebral animals, their motions are 

 generally slow, awkward, and limited. 



Their bodies are generally covered by a fold or reflection 

 of the skin, which envelops them completely, and is called their 

 mantle. In some species, the two folds of the mantle are united 

 at their edges, so as to form a complete bag, in which the body of 

 the animal is contained, opening only at one end by a sort of canal 

 or snout : in some, it extends in two opposite directions, so as to 

 answer the purpose of fins or oars. Sometimes there is only this 

 simple membranous covering j but more frequently there is a 

 hard external shell, which serves as a retreat into which the animal 

 may withdraw itself, and which it can carry about upon its back 

 in all its changes of place. These shells difiier a good deal in 

 shape, color, and texture, in different species ; and among them 

 are found some whose form, polish, and splendid tints, place them 

 among the most beautiful objects in nature. 



The MoUusca have no brain nor spinal marrow. Their ner- 

 vous system consists merely of a number of nervous masses, dis- 

 tributed in different parts of their bodies, from which are sent out 

 a great many sinall branches, that mutually unite with each other. 

 The principal of these, which is sometimes called the brain, ,is 

 situated round the oesophagus, and envelops it like a collar. In a 

 few species it is contained in a cartilaginous case. Their respira- 

 tion is not uniform. It is generally carried on by organs resem- 

 bling the gills of fishes, which are acted upon either by fresh or 

 salt water ; but, in some cases, air is respired directly from the 

 atmosphere. The circulation is always double; that is to say, there 

 is a passage of the blood through the respiratory organs, distinct 

 from that through the rest of the body. This circulation is car- 



