1 82 ELEMENTARY BIOLOGY. [CHAP. 



blood to the organs of respiration. 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 n^ 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 com- 

 partments in immediate communication with certain 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 

 irregular 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. This bailing out of the water results in the in- 

 ducing of a current which flows in by the inferior and 

 posterior cleft beneath the free edge of the branchiostegite, 

 and thus a constant circulation over the gills is secured. 



