ARTHROPOD A. 407 



a well-developed circulatory system, this loss in these animals 

 must be regarded as secondary rather than as primitive, and is 

 explained by the fact that with reduction in size the organization 

 is simplified. 



The blood may pass from the large arteries either directly into 

 the large blood sinuses of the body, erroneously called the body 

 cavity, or by a more complicated course through capillaries and 

 veins as well as through the respiratory organs. There is, on this 

 account, the greatest difference in the development of the vascular 

 system, but even in the highest forms the system is not entirely 

 closed, the blood passing to the sinuses of the body (haemocoele, 

 p, 110) and thence to the pericardium (probably arising from the 

 coalescence of veins and certainly not coelomic), from which it is 

 sucked through the ostia into the heart. 



The variations in the circulation depend upon the modifica- 

 tions of the respiratory organs, which can be described adequately 

 only in connexion with the various groups. In general it can 

 only be said that the more respiration is localized in regions and 

 organs the more nearly complete is the circulation, while with 

 respiration diffused over or through the whole body, the vascular 

 system, including even the heart, may be reduced. 



The various spaces in the body are frequently encroached upon 

 by a fat body, a kind of connective tissue whose cells, richly laden 

 with fat, serve as a store of nourishment for the animal. Besides, 

 urinary products, like uric acid, have been found in it, leading to 

 the conclusion that the fat body acts as a reservoir for excretory 

 substances before their elimination by the excretory organs. These 

 latter vary greatly in the different groups : true nephridia in Peri- 

 patus, shell glands and antennal (green) glands in the Crustacea, 

 and tubules (Malpighian tubules) connected with the intestine in 

 arachnids and insects. 



The sexual organs, which empty through ducts which are 

 apparently modified nephridia, are only rarely hermaphroditic. 

 In the bisexual species one can usually distinguish males and 

 females by external characters, such as coloration, size or form of 

 appendages, especially those used in copulation. The eggs are 

 usually large and rich in yolk, and consequently but rarely undergo 

 total segmentation. In most eggs occurs that type of partial 

 segmentation called superficial (fig. 104). While the surface of 

 the egg divides into the cells which form the blastoderm, the 

 central yolk long remains undivided a condition of systematic 

 interest since it is not known to occur outside the Arthropoda. 



