NEPHROPS. 217 
In the Arthropoda this condition is reversed. The 
coelom is reduced to a few small spaces z¢hin 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 heemoccelic system is spacious and forms the main cavities 
Colom. 
Fig. 146.—SECTION ACROSS THE ABDOMEN OF 
NEPHROPS x 2. (Ad nat.) 
Dorsal Blood-Vessel. Intestine. 
Extensor Muscle. 
Nee Cor d. Ventral Blood-Vessel. 
> Exopodite } 
Endopodite 
Swimmeret. 
of the body. It is divided by a median horizontal pericardial 
septum in the thorax into an upper cavity, the 
pericardial sinus, and a lower cavity which forms 
the general body-cavity. The Jody-cavity of the 
lobster is therefore purely heemoccelic, mostly a large venous 
cavity, but partly a small arterial* pericardial cavity. The 
heart lies dorsally in the pericardial sinus, with which tt 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 
Blood- 
Vascular. 
* Arterial in containing zrated blood; structurally it is a part of the venous 
system, 
