MOLLUSCA 2G 
is developed on the wall of the coelom, beneath each branchial 
heart. The blood, after passing through the ctenidia, is 
returned to the auricles. 
The blood contains colourless corpuscles: in the oxidised 
condition it is nearly colourless, but when venous it is bluish. 
The colour is due to haemocyanin, a substance containing 
copper, which is diffused through the serum. 
The. ctenidia are organs of considerable size. In the 
normal state their long axis is parallel with the longitudinal 
axis of the body, and they are attached throughout their whole 
length to the body-wall. The axis bears a double row of plate- 
like lamellae, which decrease towards the anterior end, thus 
giving a pyramidal shape to the organ. An afferent vein from 
the branchial heart traverses the axis and gives off branches to 
the lamellae; here the blood is aerated, and is then returned 
by an efferent vein which runs parallel and close to the former, 
this leads to the auricle. 
The nephridia are paired, right and left, but they are con- 
nected by two transverse portions, an anterior and a posterior. 
The former of these transverse communications gives off a 
diverticulum which stretches, as the unpaired nephridial sac, 
back as far as the genital gland. The ventral wall of the 
nephridia is smooth, but the dorsal wall, which is wrapped round 
the branchial veins, and into which numerous veinlets run, is 
spongy and glandular. At the anterior end of each kidney is 
a short ureter which opens to the exterior at the side of the 
anus. Near the inner end of the ureter there is a rosette- 
shaped opening covered with ciliated epithelium, which leads 
into the coelom. 
Cephalopods, which are the largest and most ferocious of all 
the invertebrates, have developed an internal cartilaginous 
skeleton, a very unusual arrangement outside the phylum 
Vertebrata. The cartilage consists of a structureless matrix, 
through which numerous cells are scattered; the cells give off 
branching processes which permeate the substance in all direc- 
tions. Nodules of this cartilage exist in processes of the 
mantle edge, and fit into corresponding depressions on the edge 
of the siphon when the mantle is closed, and also along the 
base of the lateral fins, but the most considerable developement 
