74 



Lounihiiies of lliosc near the luarg'ius can Le traced, l)ut 

 ill the centre of a large clump all trace of a cell outline 

 seems to have disappeared and a plasmodium is formed. 

 This probably has the same function as the clotting in 

 crustacean and mammalian blood, but diifers in that 

 there is no development of fibrin in the blood plasma 

 which remains fluid on standing in the air, or even on 

 heating. The same feature appears to be present in the 

 coelomic fluid of some other invertebrates (16). 



The blood as a whole appears to contain very little 

 nutritive matter. In stained sections the leucocytes in 

 tlie organs are round or oval, with retracted pseudopodia, 

 and a prominent nucleus containing smaller dark bodies. 



The central organ of circulation, the heart, is 

 situated on the dorsal posterior side of the large adductor 

 muscle, posterior to the digestive gland, against which it 

 lies. It is contained in a })ericardial cavity (fig. 1, Per.), 

 which is bounded above by a fibrous roof connecting the 

 two mantle lobes, while anteriorly it is prolonged, forming 

 two deep pouches which extend above the adductor muscle 

 and between it and the digestive gland. 



I'he pericardium is the representative of the coeloin, 

 and communicates with the exterior by a pair of excretory 

 organs, which are coelomoducts. The pericardium is 

 lined by a thin endothelium formed of flat cells. 



The heart consists of a ventricle and two auricles 

 (Hg. I'J, Ven. and Anr.). The ventricle is a large spongy 

 sac, the cavity being cut up and reduced in size by 

 numerous muscle fibres which cross in all directions. 

 AVhen contracted the size is very small. The shape is 

 roughly that of two triangles, with their bases apposed, 

 except posteriorly, and with the apices, which are 

 rounded, opening into the auricles. The ventricle has 

 grown up round the rectum and encloses it (fig. I'i, 



