ARTHROPODA 259 



structure and is many times branched. The main stem opens 

 into the cloaca. The respiratory tree is filled with water which 

 is regularly renewed from outside through the cloacal vent. 

 This is brought about in the following way. Numerous small 

 muscles connect the outside of the cloacal chamber with the 

 body wall. When these contract the cloaca expands and fills 

 with water. The cloacal aperture then closes, the walls of the 

 cloaca contract and the contents are forced into the respiratory 

 tree. The tubes of the respiratory tree also have contractile 

 walls and these by contraction again force the water out. In 

 some of the small Holothuria, which do not possess such an 

 impervious integument, the respiratory tree is absent. The 

 ambulacral system is variously developed within the group. 

 In several families the tube feet are entirely wanting. Loco- 

 motion is chiefly effected by worm-like movements of the body. 



557. Phylum VII. Arthropoda. — The Arthropoda are bi- 

 laterally symmetrical, segmented animals. They are distin- 

 guished from the Annelida by their jointed appendages. The 

 number of segments is usually not more than twenty, and they 

 are not alike (heteronomous). The body is always covered 

 with a cuticula of chitin, secreted by the epidermis. One or 

 more pairs of appendages are modified to serve as mouth parts 

 for the ingestion of food. The blood vessels open into the body 

 cavity which is also connected with the cavities derived from 

 the primitive blastula cavity. The body is typically di\dded 

 into three regions, head, thorax and abdomen. The nervous 

 system consists of a brain and ventral nerve cord as in the Anne- 

 lids. In point of numbers the phylum includes two-thirds of 

 the animal kingdom. 



558. Class I. Branchiata. — As the name impHes the 

 Branchiata are the Arthropods which are provided with gills. 

 But this is true only of the larger forms and even in some of 

 these the gills have been lost. With a very few exceptions, 

 however, the Branchiata are aquatic. The term Crustaceae 



