PBIMABY DIVISIONS AND CLASSES. 201 



which repeat each other, so that it may be divided into a 

 considerable number of segments, homologous, and more or 

 less like each other (Fig. 141). The nervous system is mo- 

 derately developed, and is composed of a double series of small 

 medullary centres, called ganglions, reunited in a longitudinal 

 chain, so as to occupy the greater part of the length of the 

 body (Fig. 139). 



The small mass formed by the first ganglions of this con- 

 nected chain is lodged in the head, and for this reason has 

 been compared to the brain of the vertebrata; but we find 

 nothing resembling the spinal marrow, for the rest of the 

 chain of ganglions is situated on the ventral surface of the 

 body under the digestive tube (Fig. 140), and the nervous 

 cords uniting them to the ganglions of the head surround the 

 gullet like a collar. All the muscles are attached to the skin, 

 and there is no internal skeleton; but the integuments, by 

 their hardness, form a sort of external skeleton, being ar- 



Fig. 141. Scolopendra. 



ranged in rings more or less moveable on each other. Thus, 

 the annulated or articulated character of these animals may 

 be seen externally; the limbs, in general, are very numerous; 

 the organs of the senses less numerous and less perfect than 

 in the vertebrate ; the blood is almost always white, and the 

 circulation very incomplete ; finally, a number of other pecu- 

 liarities are found in the structure of these animals, to which 

 we shall afterwards return; the scolopendra or centipede 

 (Fig. 141), the lobster or craw-fish, crabs, insects, &c., are 

 specimens of this primary division of the animal kingdom. 



376. Molluscous Animals. The molluscs have, like 

 the preceding, the principal organs in pairs, and symme- 

 trical ; but the body has a tendency to assume a spiral or 

 curved form, so that the mouth and anus, instead of occu- 

 pying the two extremities of the trunk of the animal, are 

 more or less contiguous. The nervous system is composed 

 essentially of ganglions, as in the anuulata ; and here also a 



