Physiology of Sagitta bipunctata. 293 



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 glasa 

 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 skin, 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. Each 

 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 in 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 diminu- 

 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 efiected 

 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 Sagittttj whilst swimming, strike 

 the water with its tail like a Cetacean. 



I. Head : — Hood. — Tlic 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 line describes on each side a great curve, passing 



