XII.] THE FRESH-WATER CRAYFISH. 129 



quantity shed shows the size of the sinus and its free commu- 

 nication with the rest of the vascular system. By cutting 

 across any one of the limbs and inserting a blow-pipe into 

 the place whence the blood wells forth, this ventral sinus 

 can be readily injected with air. A large and irregular 

 sinus is also to be found in the median dorsal region of the 

 abdomen and is freely connected with the median ventral 

 sinus. The stem of each branchia contains two canals, one 

 running along its outer and the other along its inner face. 

 The outer canal communicates, at its origin, with the median 

 ventral sinus. The inner canal opens into a passage which 

 ascends in the lateral wall of the thorax and opens, after meet- 

 ing with other ' branchio-cardiac ' canals, opposite the lateral 

 aperture of the heart. As the valvular lips of this and the 

 other apertures of the heart open inwards, the blood, when 

 the systole takes place, is driven out of the heart through 

 the various arteries, and a considerable part of the blood thus 

 propelled into the capillaries is collected by the median ventral 

 sinus and thence, passing through the gills, eventually returns 

 to the heart, which is therefore, like the heart of Anodon, 

 a systemic and not a branchial heart. But whether the 

 whole of the venous blood takes the same course, or whether 

 some of it returns from the dorsal sinuses directly to the 

 pericardium, is a question which is not decided. Nor is it 

 certain whether the so-called pericardium is to be regarded 

 as one cavity, or whether the fibrous bands, which connect 

 the heart with its walls, may not subdivide it into compart- 

 ments in immediate communication with certain of the car- 

 diac apertures, and not with the rest. 



In the Lobster, from which the blood is readily obtained 

 in quantity, it is a nearly colourless fluid, which usually has 

 a faint neutral tint. It readily coagulates, a tolerably firm 

 clot separating from the serum. It contains nucleated cor- 

 puscles, devoid of any noticeable colour, which throw out very 

 M. 9 



