170 



HOMOLOGY 



where the food material is taken up. Such a circulation of 

 blood is spoken of as an open circulation as opposed to the 

 dosed circulation of organisms like the earthworm which have 

 both arterial and venous capillaries so that the blood is always 

 within specialized blood vessels. The body cavity of the lobster, 

 therefore, is quite different from the coelom of an earthworm 

 and other animals. It is not lined by endothelium and does 



FIG. 70. Transverse section through the thorax of a lobster to show the rela- 

 tion of the gflb to the branchial chamber, the haemoooei, and the chamber of the 

 heart. (Modified after Lang.) 



not contain the opening of the excretory organs (nephridia) nor 

 do its walls give rise to the germ glands. It corresponds rather 

 to a large blood sinus, and for this reason is termed a haemocod 

 and not a coelom. The real coelom of these forms is limited 

 to the small cavities of the nephiidia and the germ glands. 



Gills. The blood mixed with digested food, in the haemocoel, 

 passes slowly into the gills where it is aerated. The gills are 

 pockets of tissue derived from the epithelium, drawn out in the 

 form of long triangular pyramids with broad bases and pointed 



