NEPHROPS, 



217 



In the ArtJiJ'opoda this condition is reversed. The 

 coelom is reduced to a few small spaces within the meso- 



^ derm, such as the cavity of the gonads and of 



green gland, is either indifferent or excretory, 



and has lost its motor function, whereas the blood-vascular 



or haemocoelic system is spacious and forms the main cavities 



Fig. 146. — Section across the Abdomen of 

 Nephrops X 2. (Adnat.) 



Dorsal Blood-Vessel. 

 Extensor Muscle. 



Intestine. 



Nerve Cord, 



Ventral Blood-Vessel 



Exopodite -v | 

 Endopodite J .§ 



Blood- 

 Vascular. 



of the body. It is divided by a median \iOx\zon\.2\ pericardial 

 septum in the thorax into an upper cavity, tlie 

 pericardial sinus ^ and a lower cavity which forms 

 the general body-cavity. The body-cavity of the 

 lobster is therefore purely haemocoelic, mostly a large venous 

 cavity, but partly a small arterial^ pericardial cavity. The 

 ^iieart lies dorsally in the pericardial sinus, with jvhich it com- 

 municates by six valves ; on contraction it drives the blood 

 forwards and backwards by main arteries. Forwards there 

 is a median ophthalmic artery, paired antennary and 



* Arterial in containing aerated blood ; structurally it is a part of the venous 

 system. 



