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 points, which 

 are turned towards the mouth. When the animal seizes its prey, 

 these hooks, by a simultaneous retraction of the hood, first point 

 upwards separating from one another, and are then lowered to- 

 wards the object which the animal wishes to seize. Besides these 

 spines, the head has two rounded prominences, situated on its 

 anterior margin, and a simple series of very small, straight, sharp, 

 and horny spines. Another similar series of spines is found on 

 two ligulse 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 

 canals, 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 th^ 

 excretory ducts of the ovaries 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*. Each aperture, as we shall observe in detail further 

 on, communicates with one of the seminal cavities above-men- 

 tioned, and assists in the secretion of the semen. The anus is 

 situated nearly at the same height as the apertures of the excre- 

 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 Sw 

 gitta. 



Y2 



