128 A COURSE ON ZOOLOGY. 



To this class belong all the animals named by Cuvier 

 articulates because their bodies consist of rings having 

 articulated ends, that is, mobile pieces or articles 

 adapted for locomotion and for nutrition. According to 

 the species the solid rings or segments are furnished 

 each with one or with several pairs of appendages. 



The bodies of these animals present three distinct re- 

 gions, the head, the thorax, and the abdomen, 



The head is the front part ; it is formed of several 

 rings, but these are so closely united together that their 

 limits cannot be distinguished ; however, the number of 

 appendages indicates the number of rings composing the 

 head. Among these appendages, some placed near the 

 eyes are called antennae ; others near the mouth aid in 

 the functions of nutrition. The head contains the or- 

 gans of 'sense, and also an important nerve-centre com- 

 posed of the cerebral ganglions. 



The thorax is composed of rings that are sometimes 

 firmly joined together, sometimes entirely separate ; in 

 the latter case they are usually three in number. This 

 region is generally quite separate from the head, but 

 sometimes it forms with the latter but a single piece, 

 which is then called cephalo-thorax. 



The rings that form the abdomen are distinct and 

 vary considerably in number. 



Both the thorax and the abdomen may have appen- 

 dages, but the limbs are specially related to the pieces of 

 the thorax. 



We have said that the rings of the body and those of 

 the members form in the articulates an external skeleton 

 that we have compared to the internal skeleton of the 

 vertebrates. It must not, however, be supposed that 

 there is any real resemblance between the two sub- 



