290 ARTHROPODA. CRUSTACEA 



gills, but in some cases respiration takes place through 

 the general surface of the body. The chitinous exo- 

 skeleton is frequently hardened by a calcareous deposit, — 

 hence the name Crustacea. Segmentation is usually well 

 marked, but in the Ostracoda is shown by the appendages 

 only. The exoskeleton of a segment consists of a dorsal 

 part, the tergum, and a ventral part, the sternum. Three 

 regions may be distinguished in the body : — the head, the 

 thorax, and the abdomen; but in the lower forms the 

 abdomen is often not clearly marked off from the thorax. 

 In the head there are five segments fused together, but ex- 

 ternally these (except in Trilobites) are indicated only by 

 the appendages. The number of segments in the thorax 

 and abdomen is variable in the lower Crustacea, but is con- 

 stant in the Malacostraca. In many forms some or all of the 

 segments of the thorax fuse with those of the head, forming 

 a cephalothorax. In many Crustacea there is a dorsal shield 

 or carapace which covers part, or sometimes the whole, of 

 the body, and originates as an outgrowth from the pos- 

 terior margin of the dorsal part of the head. The head 

 usually bears five pairs of appendages, viz. : — two pairs of 

 feelers (the antennules and antennae), one of mandibles, and 

 two of maxillae ; the first two pairs are in front of the 

 mouth. The thorax is also provided with appendages, 

 and often the abdomen too. The mandibles and maxillae, 

 and frequently some of the anterior thoracic appendages, 

 serve as jaws. The Crustacean appendage is typically 

 biramous, consisting of a basal part (the protopodite) bear- 

 ing two branches — the inner called the endopodite, and 

 the outer termed the exopodite. The protopodite usually 

 consists of two segments — a proximal or coxopodite, and a 

 distal or basipodite. The mouth is on the under surface 

 of the head, and the anus is on the last segment (the 



