Physiology of Sagitta bipunctata. 301 
the ventral surface of the trunk, and like the preceding one, just 
beneath the skin; it must be sought for between the head and 
the first pair of fins, but nearer to the latter. It is ovoid, elon- 
gated, swollen, and in adult individuals is nearly a millimetre 
and a half long. It consists of a medullary substance or intense 
white nucleus, and of a cortical layer of a fainter white. This 
last layer is composed of a multitude of ganglionic globules. 
This nerve furnishes four principal branches, which, in their 
course, proceed along the ventral surface of the animal. Of these 
branches two are anterior ; these are the pharyngeal commissures ; 
the two others are posterior. Beside these branches, this gan- 
glion furnishes a great number of nervous filaments, which de- 
tach themselves from it on all sides. 
The two pharyngeal commissures proceed from the anterior 
extremity of the ganglion, at first diverging from it; but they 
soon proceed in a straight line and parallel to the head. They 
attach themselves strongly to the skin, are very flattened through- 
out their course, and become more and more narrow in propor- 
tion as they approach the head. When they reach it, each of 
them follows the lateral and upper insertion of the cephalic hood, 
creeping immediately under the skin ; they form a kind of beau- 
tiful arcade, and after becoming extremely delicate, unite with the 
cephalic ganglon. 
The two branches furnished by the posterior part of the ven- 
tral ganglion are larger, but shorter than the pharyngeal com- 
missures, for they scarcely pass the first pair of fins; they also 
detach themselves from the ganglion, diverging from it, but soon 
take a parallel course backwards. At their posterior extremity 
they furnish a multitude of ramifications which at first remain at 
the side of one another, but subsequently exhibit greater diver- 
gence and assume something of the form of a horse-tail. 
From the external margins of all the branches of the ventral 
ganglion a number of nerves separate; these ramifications, like 
those proceeding directly from the ventral ganglion, form all of 
them a curve on ascending toward the dorsal surface of the ani- 
mal, and during their course become more and more divided, and 
furnish, by adhering and anastomosing, a fine and very compli- 
cated nervous network beneath the skin of the trunk. 
Eyes.—We have already said that the optic nerves arise from 
the posterior cephalic nerves. Each optic nerve has its origin at 
the external margin of the branch which furnishes it; it then 
swells into a rounded ganglion, on which the eye is as it were set. 
The ganglion and the eye are placed in a peculiar closed cayity, 
hollowed in the skin of the head. The eye is much smaller than 
its ganglion ; it is spherical, and enveloped in a pigment of a deep 
colour. When this eye is examined with the microscope there is 
