174 HOMOLOGY 



later from the main arteries into the general cavity of the body, 

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

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

 closed 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 lob- 

 ster, therefore, is quite different from the coelom of an earth- 

 worm and other animals. It is not lined by endothelium, and 



branchial 



chamber 



haemocoel^. 



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

 tion of the gills to the bronchial chamber, the haemocoel, and the chamber of the 

 heart. (Modified after Lang.) 



does 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 

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

 limited to the small cavities of the nephridia 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 



