242 ARTHROPODA [CH. 



Peripatus, the most primitive Arthropod, has in each segment 

 excretory organs in the form of coelomiducts, each of which opens 

 internally, not into a general coelom, but into a small sac which 

 *s really a special part of the coelom not communicating with 

 any other coelomic space but belonging only to the excretory organ 

 which opens into it. The main body-cavity is a haemocoele. Mol- 

 lusca also retain a pair of such coelomiducts or in the case of 

 Nautilus two pairs. These organs open into special coelomic 

 spaces which are as a rule small, the more spacious cavities of 

 the body being haemocoelic. Crustacea also retain coelomiducts, 

 those of the fourth segment shell glands being persistent in the 

 Entomostraca, whilst those of the second segment green glands 

 persist in the Malacostraca. It seems probable that the sacs 

 at the inner ends of these glands represent coelomic spaces. But 

 even amongst Crustacea, in certain Amphipods for instance, we 

 find the transference of the function of the coelomiducts to out- 

 growths of the alimentary canal. In Arachnids the coelomiducts 

 or coxal glands show very varying degrees of development, but 

 on the whole they are tending to die out and to be replaced by 

 Malpighian tubules or diverticula of the intestine. Finally in 

 Myriapods and Insects, where the true coelom is reduced to its 

 minimum and where the tracheal mode of respiration attains a very 

 high degree of development, all traces of coelomiducts have dis- 

 appeared and the waste nitrogenous matter is excreted solely by the 

 Malpighian tubules. 



These are very fine long blind tubes, so fine as to be but 

 just visible to the unaided eye, 60 80 in number, arranged in 

 six bundles which open into the beginning of the narrow part of 

 the proctodaeum from which they are outgrowths. They float in the 

 blood, winding about amongst the abdominal viscera (Fig. 99). They 

 contain crystals, probably of urate of soda, which are taken up from 

 the blood and which leave the body through the intestine. 



The nervous system of the cockroach is constructed on the same 

 plan as that of the other segmented Invertebrates with which we 

 have had to do. There is a large supra-oesophageal ganglion or 

 brain giving off commissures, which encircle the oesophagus and 

 unite below in a sub-oesophageal ganglion. Together these occupy 

 a considerable portion of the cavity of the head. The supra- 

 oesophageal ganglion supplies paired nerves to the eyes and to the 

 antennae and is thus the sensory centre. The sub-oesophageal 

 ganglion supplies the mandibles and both pairs of maxillae. From 



