406 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
boundaries of those near the margins can be traced, but 
in 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 difters 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 
the 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 pericardial 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 
und between it and the digestive gland. 
The pericardium is the representative of the coelom, 
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 
(fig. 18, Ven. and Aur.). 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. 
When 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 
erown up round the rectum and encloses it (fig. 15, 
