MOLLUSC A 217 



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 haemoeyanin, 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 



