STRUCTURE OF MOLLUSCA. 7? 



wo opposite directions, so as to answer the purpose of fins 

 or oars. Sometimes there is only this simple membranaceous 

 covering ; but more frequently there is a hard external shell, 

 which serves as a retreat into which the animal may with- 

 draw itself, and which it can carry about upon its back in 

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

 shape, color, and texture, in different species : and among 

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

 place thejn among the most beautiful objects in nature. 



The Mollusca have no brain nor spinal marrow. Their 

 nervous system consists merely of a number of nervous masses, 

 distributed in different parts of their bodies, from which are 

 sent out a great many small branches, that mutually unite with 

 each other. The principal of these, which is sometimes 

 called the brain, is situated round the esophagus, and envel- 

 ops it like a collar. In a few species it is contained in a car- 

 tilaginous case. Their respiration is not uniform. It is gen- 

 erally carried on by organs resembling the gills of rishes, which 

 are acted upon either by fresh or salt water ; but, in some 

 cases, air is respired directly from the atmosphere. The cir- 

 culation 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 carried on 

 by either one or more hearts. When there is only one, it is 

 situated so as to receive the blood from the gills, and circulate 

 it through the body. When there are two, the second is 

 situated so as to circulate through the gills the blood coming 

 from the body. In some species, there are three hearts ; and 

 in this case, as there are two sets of gills, a distinct heart is 

 devoted to each. The blood in the Mollusca is thin, of a 

 bluish white, and always cold. 



The organs of digestion vary very much. Sometimes there 

 are organs for mastication, and sometimes not. Some species 

 have only a single stomach, and others have several ; the 

 structure of this organ, in some species, very much resembling 

 that of the gizzard of birds. In some species there are four 

 stomachs, which bear a great analogy to those of the rumina- 

 ting animals, and have been supposed to answer a similar pur- 

 pose. In the intestines there is as great a variety. 



This class is divided into several orders, according to the 

 general form and structure of the species composing it. A 

 few of the most important particulars that distinguish them 

 will be pointed out. 



Tn the first order, containing the cuttle-fish, squid, nauti- 

 7* 



