1 82 ELEMENTARY BIOLOGY. [CHAP. 



blood to the organs of respiration. The central cavity of 

 the carapace is a vascular one, receiving its blood from a 

 great sinus which surrounds the liver and stomach, and in 

 part, it is believed, from the arteries of the anterior 

 extremity ; the blood, on leaving the carapace, is collected 

 into two great trunks which enter the pericardium postero- 

 dorsally. It is uncertain 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 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, which throw out very long pseudopodial prolon- 

 gations, 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 move- 

 ment forwards and backwards, scooping out the water 

 in the branchial chamber. This action results in the 

 inducing 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. 



