280 



COMPARATIVE ANATOMY. 



ments. The thorax is composed of three segments : prothorax, 

 mesothorax, and metathorax, answering to the three anterior trunk 

 segments of the Myriapoda. The typical number of segments in 

 the hind-body is ten or eleven. The thorax and the abdomen 

 together form the trunk, which may be compared with the trunk 

 of the Symphyla. Among the Apterygota the Thysaiiura possess 

 ten abdominal segments, and the CoUembola a varying number, but 

 always less than ten. In the Pterygota the number of abdominal 

 segments in the adult animals varies, and is generally less than ten. 

 This diminution is caused by the fusing of those segments which 

 are connected with the genital apparatus and lie in front of the 



Fig. 21, A — D. 

 The Nervous Systems of Four Species of Diptera, 

 to demonstrate their various degrees of concentration. 

 A, Non-concentrated nervous system of CJiirononms plumosus, with three 

 thoracic and six abdominal ganglionic masses. ^i5, Nervous system of Einpis 

 sterco7-ea, with two thoracic and five abdominal ganglionic masses. — C, Nervous 

 system of Tabanus bovinus, with one thoracic ganglionic mass, and the abdo- 

 minal ganglia moved towards each other. — D, Nervous system ot Sarcophaga 

 carnaria. All the ganglia of the ventral chord except the infra-oesophageal 

 ganglion, which always remains separate, are here united into one single 

 thoracic ganglion mass. — (After E. Brand.) 



