200 PHYSIOLOGY OF THE INVERTEBRATA. 



and then passes through proper vessels to the heart. It 

 will be seen that the circulation in the higher forms is semi- 

 vascular and semi-lacunar. 



The circulatory apparatus of Astacus is well defined. The 

 heart is situated dorsally and behind the stomach. It is 

 surrounded by the so-called pericardium, which is in reality a 

 blood-sinus ; consequently the heart is suspended in a blood- 

 sinus. There are six apertures in this organ provided with 

 valves which open inwards. These allow the blood to enter 

 the heart during the diastole, and prevent its egress, except 

 by the arteries, during the systole. There are six arterial 

 trunks provided with valves at their commencement, their 

 object being to prevent the regurgitation of the blood. 

 These arteries ramify minutely, but the capillary system has 

 not been investigated with anything like satisfaction. So 

 far as is known, the blood passes from the arteries into the 

 lacunjB and into the perivisceral cavity. From these lacunee 

 it ultimately finds its way to the branchi^ and heart. 



Eeverting once more to the heart of the Crustacea, Sir 

 Eichard Owen, P. E.g., says that " we may trace in the heart 

 of these animals a gradational series of forms, from the 

 elongated median dorsal vessel of Limulus, to the short, 

 broad, and compact muscular ventricle in the lobster and the 

 crab. In all the Crustacea the heart is situated immediately 

 beneath the skin of the back, above the intestinal tube, and 

 is retained in situ by lateral pyramidal muscles. 



" In the Entomostraca* and in the lower, elongated, slender, 

 many-jointed species of the Edriophthalmous Cnistacea, the 

 heart presents its vasiform character. It is broadest and 

 most compact in the crab." 



The Polyzoa. 



The perivisceral cavity contains a nutritive fluid. This is 

 kept in constant motion by- the action of cilia with which the 



* The Entomostraca include the Phyllapoda, Cladocera, Ostracoda, and the 

 Copepoda. 



