SEA-FISHERIES LABORATORY. 443 
periphery, and terminates at the point where the retina 
comes in connection with the sides of the optic vesicle. 
The tapetum is composed of large irregular cells arranged 
rather irregularly in two layers, and in many cases the 
margins are difficult to define owing to the dense pigment 
contained in them. ‘The cells contain each a nucleus, 
often obscured by the pigment, which is present in the 
form of rather large granules, in shape something like 
those found in the digestive gland cells, but of quite 
a different colour—a dark brown-red. The pigment 
eranules are much larger than those found in the cells of 
the iris. 
There only remains to be described the inner wall of 
the eye vesicle against which the tapetum rests. This is 
known as the sclerotica (fig. 29, Scl.), and is a differen- 
tiation of the connective tissue of the stalk, which becomes 
tough and hyaline and stains rather more deeply than the 
surrounding tissue. It passes into the septal membrane 
at the edge of the retina, so that septal membrane and 
sclerotica together form a closed vesicle in which is 
situated retina, argentea and tapetum, the whole being 
known as the ommateal sac. 
Optic Nerves—The eyes are innervated by optic 
nerves, Which arise from the circumpallial nerve and pass 
through the centre of the eye stalk (fig. 29, Op.n.) until 
the eye vesicle is almost reached. Here the nerve divides 
into two branches, one of which, the inner nerve 
(Op.n.7.), continues its course until immediately below 
the sclerotica, where it breaks up into many bundles of 
nerves, which radiate from this point and ascend on all 
sides of the eye vesicle to reach the periphery of the 
retina where they are continuous with the rod cells. The 
other branch, the outer nerve (fig. 29, Op.n.o.), passes 
distally on the shell side of the eye stalk, and, as already 
