INVERTEBRATA. 441 



Annelida, but they differ in the absence of chaetae, the 

 character of the appendages, and the small number of the 

 segments. Processes of the cuticle are common, but these 

 setae are never imbedded in sockets like the chaetae of 

 Chaetopoda. The internal characters are the reduction of 

 the coelom, which is represented only by the cavities of 

 the generative organs and the terminal sacs of the excre- 

 tory organs, the absence of true nephridia, and the ostiate 

 heart surrounded by a blood space representing the venous 

 vessels. 



The Arthropoda include the following classes : Crustacea, 

 Arachnida, Prototracheata, Myriapoda, and Insecta. 



CLASS: CRUSTACEA. 



Arthropoda with two pairs of antennae ; at least three 

 pairs of appendages converted into jaws, i.e. mandibles 

 and two pairs of maxillae ; these five pairs of appendages 

 are attached to the compound head formed by the union 

 of the corresponding segments ; the rest of the body 

 usually divisible into two regions, the thorax or pereion, 

 and abdomen or pleon. The dorsal wall of the head is 

 usually prolonged backwards to form a shell which covers 

 more or less of the body, and in the higher forms fuses 

 with some or all of the thoracic segments to form the 

 cephalo-thorax. Respiration, with few exceptions (wood- 

 lice), aquatic, branchiae consisting of outgrowths of the 

 body wall or of the limbs. There is a median eye which 

 often disappears in the adult, and a pair of compound 

 eyes often raised on peduncles. Astacus is one of the 

 order Decapoda which is distinguished by the great en- 

 largement of the five posterior pairs of thoracic legs and 

 the modification of the three anterior pairs into maxilli- 

 peds. Forms like Astacus with well- developed pleon form 

 the sub- order Macrura or long- tailed Decapods, con- 

 trasted with the crabs or Brachyura, in which the pleon 

 is rudimentary and the cephalothorax is broad and short. 

 The Decapoda, with other orders, such as the Arthrostraca 

 (including wood-lice, sand-hoppers, etc.), form the sub- 

 class Malacostraca, distinguished by having with few 

 exceptions 19 segments, while the remaining Crustacea are 



