xii.] THE FRESH- WATER CRAYFISH. 135 



course, or whether some of it returns from the dorsal sinuses 

 directly to the pericardium, is a question which is not de- 

 cided. 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 compartments in immediate communication with cer- 

 tain of the cardiac 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 long pseudopodial prolongations, and thereby take an 

 irregularly stellate form. 



It has been seen that the respiratory organs, or branchiae, 

 are lodged in a chamber situated between the branchiostegite 

 externally, the lateral walls of the thoracic somites internally, 

 and the bases of the thoracic limbs below; and that there 

 is a narrow interspace between the free edge of the bran- 

 chiostegite and the latter. At the anterior end of the cham- 

 ber, a funnel-shaped passage leads to the anterior opening 

 mentioned above, and, in this passage, the scaphognathite 

 lies like a swing door. 



During life, the scaphognathite is in incessant movement 

 forwards and backwards, scooping out the water in the bran- 

 chial chamber through its anterior aperture at every forward 

 motion. The place of the water thus thrown out is taken by 

 water which flows in by the inferior and posterior cleft be- 

 neath the free edge of the branchiostegite, and thus a constant 

 current over the gills is secured. Each branchia is somewhat 

 like a bottle-brush, having a stem beset with numerous fila- 

 ments; and the blood contained in the vessels of the latter 



