358 ARTHROPODA 



The blood may pass from the large arteries either directly into the 

 large blood sinuses of the body (hiemocoele, p. no), erroneously called 

 the body cavity, or by a more complicated course through capillaries and 

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

 account, great differences in the development of the vascular system, 

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

 passing to the haemocoele and thence to the pericardium (probably 

 arising from the coalescence of veins), from which it is sucked through 

 the ostia into the heart. The variations in the circulation depend 

 chiefly upon the modifications of the respiratory organs, which can be 

 described adequately only in connexion with the various groups. In 

 general it can 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. 



Anatomy and embryology show that the coelom is greatly reduced in the 

 arthropods and is represented by the cavities of the reproductive and nephridial 

 organs, and in the decapods by a pair of thin-walled sacs connected with the 

 green glands. Sinus-like enlargements of the blood-vessels like the haemocoele 

 occur in several annelids (Magelona). 



The excretory organs are of two different types. The segmental 

 organs of Peripatus, the shell and green glands of Crustacea, the coxal 

 glands of Acerata, and the head glands of insects are modified nephridia 

 in which the nephrostomes have been converted into small sacs. The 

 other type includes the Malpighian tubules of insects, diverticula of the 

 alimentary canal, opening at the junction of intestine and rectum. 



The sexual organs, which empty through ducts which are apparently 

 modified nephridia, are rarely hermaphroditic. In the bisexual species 

 one can usually distinguish males and females by external characters, 

 such as size, coloration, or form of appendages, especially those used in 

 copulation. The eggs are usually large and rich in yolk, and conse- 

 quently but rarely undergo total segmentation. Instead there is a super- 

 ficial segmentation (fig. 106), in which the surface of the egg divides into 

 the cells which form the blastoderm; the central yolk long remains un- 

 divided a condition of systematic interest since it is not known to occur 

 outside the Arthropoda. The cases of discoidal and unequal segmenta- 

 tion are apparently derived from the superficial. 



In accordance with the high organization, reproduction by fission or 

 budding never occurs, but parthenogenesis and pa?dogenesis do. In 

 some parthenogenesis has a certain relationship to the life history. In 

 lower Crustacea and in Aphides (plant lice) it allows the species to spread 



