II. ACERATA 



3S7 



Class II. Acerata. 



The animals comprising this group were formerly divided among the 

 tracheates (p. 359) and the Crustacea, but although differing widely in 

 respiration, the forms included are closely allied in structure and develop- 

 ment and present many differences from both Crustacea and Insecta. 

 The former views were based upon the view that trachea; wherever found 

 were homologous structures. 



In the Acerata the body is usually chvided into cephalothorax and 

 abdomen, though in some cases (mites) the two regions become fused. 



The cephalothorax consists of six somites which 

 always bear appendages, arranged in a circle 

 around the mouth, the basal joints of one or 

 more pairs frequently serving as jaws. None 

 of these appendages are like antennce (whence 



Fig. 415. Fig. 416. 



Fig. 415. — Digestive tract of Ctenida ciEnientaria (from Lang, after Duges). a, 

 abdomen; an, anus; da, dl, diverticula ('liver') of midgut; g, brain; vh, rectal bladder 

 (stercoral pocket) ; vm. excretory tubules. 



Fig. 416. — Schematic long section of lung of spider (after Mac Cleod). a, air space; 

 c, chitin layer; //, lung leaves; p, posterior wall of lung sac. 



the name of the group). The abdomen consists of a varying number of 

 somites, all of which may be free, or may be fused. These abdominal 

 somites bear appendages in the embryo, but in the adults (except the 

 Xiphosura) these are usually lost or so modified that their existence is 

 only recognized by a study of development. 



The alimentary canal is straight, without marked enlargements, and 

 lacks a chewing stomach. The liver is large and opens into the intestine 

 by two or more pairs of ducts. The nervous system has some or all of its 

 ventral ganglia arranged in a ring around the oesophagus, and in many 



