Physiology of Sagitta bipunctata. 291 
of the head is furnished at its edges with rather a broad ligula, 
but growing narrow as it rises. If the hood covers the head, 
and consequently the hooked spines, the latter, on both sides of 
the head, approach and touch one another at their pomts, which 
are turned towards the mouth. When the animal seizes its prey, 
these hooks, by a simultaneous retraction of the hood, first pomt 
upwards separating from one another, and are then lowered to- 
wards the object which the animal wishes to seize. Besides these 
spies, the head has two rounded prominences, situated on its 
anterior margin, and asimple series of very small, straight, sharp, 
and horny spines. Another similar series of spines is found on 
two ligule situated on two prominences on the under surface of 
the head. Nearly in the middle of the upper surface of the head, 
two very small oculiform, blackish points are observed, which have 
been already indicated by MM. Quoy and Gaimard. 
It is well to remark here, that the apertures of the excretory 
eanals, of the organs of generation and the anus are on the pos- 
terior half of the body. The animal is evidently hermaphrodite, 
for it possesses two ovaries, one on each side, and two cavities or 
cells in the posterior part of the abdomen or tail, and which serve 
for the secretion of the seminal fluid. The two apertures of the 
excretory ducts of the oyaries are at the base of the pair of inter- 
mediate fins, on the surface of the body, opposite to one another. 
Immediately in front of the base of the caudal fin there exists, on 
each side, a rounded and blackish prominence, which presents an 
aperture in the form of a fissure, directed obliquely from above 
downwards*. Fach aperture, as we shall observe im detail further 
on, communicates with one of the seminal cavities above-men- 
tioned, and assists in the seeretion of the semen. ‘The anus is 
situated nearly at the same height as the apertures of the exere- 
tory canals of the ovaries; but it is placed on the mesial ventral 
line of the body. 
The length of the most developed individuals is nearly two 
inches and a half ; the smallest which I have observed were about 
two lines and a half long; they were consequently only half the 
size of those observed by MM. Quoy and Gaimard. They re- 
sembled however, in all other respects, adult individuals. 
This animal swims with great swiftness, and justifies the name 
which has been given it by the French naturalists. When it is 
touched after a long repose, it darts away suddenly with the ra- 
pidity of an arrow. During these movements, the fins appear to 
be wholly inactive. Indeed, from their structure, these parts do 
* It is these two parts, these two blackish points, but not of this colour in 
all individuals, which have caused its specific name to be given to our Sa- 
gitta. 
Y2 
