35S ARTUROrODA 



The blood may pass from tlic large arleries either directly into the 

 large blood sinuses of tl\e bod\' i^ha-nioca-ie, p. t lo), erroneously called 

 the bodv ca\'iiv, or b\" a more complicated course through capillaries and 

 A-eius as well as through ihc respiratory organs. There are, on tliis 

 account, great dilTerences in the de\elopment of the \ascular system, 

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

 passing to the hxmocade and thence to the pericardium (probably 

 arising from the coalescence of \eins), Irom which it is sucked tiirough 

 the ostia into the heart. The \ariations in the circulation depend 

 chiellv upon the modifications of the respiratory organs, wliich can be 

 described ade(|uately only in connexion with the \arious 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 

 dit'fused o\'er or through the whole body, the vascular system, including 

 even the heart, may be reduced. 



Anatomy and embryology show fliat (lie co?Iom is greatly reduced in llie 

 arthropods and is represented l)y the ca\alies of the reprddacliw ami ncphridial 

 organs, and in the decapods liy a pair of thin-walled sacs connected \\ilh the 

 green glands. Sinus-like enlargerneiits of the blood-vessels like the h;vinoea-le 

 occur in several annelids {Ma^cljiu). 



The excretory orgatrs are of two different types. The segmental 

 organs of rcripaliis, the shell and green glands of Crustacea, tiie co.xal 

 glands of Acerata, and the head glai^ds of insects are modilied nephridia 

 in wliich the irephrostomes ha\-e been converted into smttil sacs. The 

 other type includes the JNIalpighian tubules of insects, di\erticula of the 

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



The sexual organs, which empty through dticls \\hich are ai)]iarently 

 modilied nephridia, are rarely hermtiphrotlilic. li\ the bisexutil species 

 oi^e can usually tlistinguish males and females b\- exteriKtl 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 lutt rarely undergo total segmentation. Instead there is a super- 

 ficial segmentation (lig. loo), in which the surface of the egg divides into 

 the cells which form the blastoderm; (lie central \o\k long remains un- 

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

 outside the .Vrlhropoda. The cases of di.scoidal and uneipial segmenta- 

 tion are apparently deri\ed from the superficial. 



In accordance with the high organization, reproduction by lission or 

 budding never occurs, but parthenogenesis and jKedogenesis do. In 

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

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



