OP ANIMALS. 33 



two similar sides by a median line; but it is subdivided into 

 two others very different. In the mollusca the body is not 

 divided into segments, and there are no articulated feet, for 

 they are inarticulate. The other symmetrical animals, on the 

 contrary, are articulate, i. e. their body is divided into seg- 

 ments, moveable upon each other, and their limbs, when they 

 have any, are divided into several parts by articulations. We 

 already discover the articular arrangement in the cirrhipoda, 

 which properly belong to themollusca; the rudiments of it are 

 also perceived in iheci/lindricalechinodermata^nd in worms, 

 but this kind of form more particularly belongs to the annelides, 

 insecta, Crustacea, and arachnides, which for this reason are 

 called articulated animals, and to all the vertebrated animals. 

 Thus we may in conclusion refer the animal forms to the fol- 

 lowing: the symmetrical or binary form, with or without ar- 

 ticulations, the radiated form, and the simple forms of a glo- 

 bule, of a filament, &c. &c. 



18. The external configuration of animals presents also 

 other differences. The body is divided into a trunk, a central 

 part, which contains the organs essential to life, or in other 

 words, the viscera, and into appendages, parts generally des- 

 tined for motion and sensation. The trunk is divided into the 

 trunk proper, or the middle part, and into the extremities, 

 the head and tail; the trunk itself is sometimes subdivided 

 into abdomen and thorax. The head is the part which 

 contains, besides the mouth, the principal nervous expansion, 

 or the brain, and the organs of the special senses. The tho- 

 rax, in the articulated animals, is the part of the trunk to 

 which the limbs are attached; in the vertebrata it is that which 

 contains the heart and lungs. The abdomen always contains 

 the principal organs of digestion and of generation. These 

 various parts of the trunk, which do not all constantly exist, 

 present diverse varieties. 



In the radiated animals, in the acephalous mollusca, and 

 in the intestina and annelides, the trunk is reduced to its mid- 

 dle part, consists of a single cavity, which contains all the or- 

 gans. In the cephalous mollusca there is a distinct head; 

 the same is the case with the insecta, Crustacea, and arach- 



