Physiologtj 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 ganghon. 



The two branches furnished by the posterior part of the ven- 

 tral ganglion are larger, but shorter than the phar5mgeal 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 cavity, 

 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 



