Physiology of Sagitta bipunctata. 295 
fascicles of fibres are perceived, advancing parallel to one another 
in the direction which I have indicated. These fibres, which, in 
their course from the base to the margin of the fin, become more 
and more slender as they advance, adhere so intimately to the 
fundamental substance, that they cannot be detached by any 
means, neither by compression nor by sliding one plate of glass 
over another, between which a fin had been previously placed. I 
must state here that these fibres in no manner resemble the 
muscular fibres. 
Muscular layer —The fascicles of muscular fibres, placed im- 
mediately beneath the skm, and which aid the animal’s locomo- 
tion, form two bands extending the whole length of the body, 
but separated from each other by a considerable space. These 
two bands are placed opposite to one another ; one on the dorsal 
surface, and the other on the ventral surface of the animal. Hach 
of them however occupies a small portion of the two lateral sur- 
faces. From the isolation of these bands, a considerable interval 
on each side results, where the skin is not furnished with a fleshy 
layer,—a circumstance which it is not easy to perceive in the living 
animal, on account of the transparency of all the tissues in their 
normal state, but which becomes evident after the muscular 
bands have been rendered turbid by their immersion im alcohol. 
The width of these muscular bands diminishes in the same pro- 
portion as the body recedes towards its two extremities. This is 
equally true for the intervals between them. To speak strictly, 
each band is resolved into two lateral halves, which remain closely 
connected with one another ; so that the number of the muscular 
bands properly speaking is four. They are composed solely of 
fascicles of longitudinal fibres, arranged in several layers super- 
posed and striated horizontally, like the primitive muscular fas- 
cicles of Insects and Crustacea. For this reason the animal 
is only capable of bending and extending the body ; any dimimu- 
tion of the volume of the body in the direction of its transversal 
diameter is impossible. From what we have said respecting the 
direction of the muscular fascicles, it will be understood that all the 
motions from above downward, or from below upward, are effected 
by these animals with greater energy than the lateral motions. 
The observations of MM. Quoy and Gaimard perfectly agree with 
our own ; for they have seen the Sagitta, whilst swimining, strike 
the water with its tail ike a Cetacean. 
I. Head :—Hood.—The hood is formed by the duplicature of 
the teguments of the head; the internal lamella appears to be 
more delicate than the external. Its insertion on the head follows 
the course of a line which, starting from the middle of the upper 
surface of the head, is directed a little behind its anterior mar- 
gin. This lme describes on each side a great curve, passing 
