18 BUSK, ON THE STRUCTURE AND 



tuted and those by which the " fins" are strengthened. With 

 reference to the latter, Krohn remarks that after repeated 

 observation he is convinced that these fibres are closely allied 

 to the setcB of Annelids. Like these they are flexible to a 

 certain extent, and are readily broken into pieces. In form 

 they exactly resemble the simple or capillary setce. He 

 remarks also, though this hardly accords with my own ob- 

 servation, that they are merely loosely imbedded in the 

 homogeneous substance of the fin ; since they may often, in 

 otherwise uninjured " fins," be seen bare for a considerable 

 extent. At any rate their connection with the substance of 

 the fins is by no means so close as he was formerly inclined 

 to believe.* 



Immediately beneath the integument is placed a layer of 

 longitudinal muscles extending uninterruptedly from the head 

 to the caudal portion of the body. These muscles are dis- 

 posed in two broad bands, one situated on the dorsal and the 

 other on the abdominal aspect, and separated on each side by 

 a clear space, which is brought more distinctly into view, as 

 Krohn remarks, when the muscles themselves are rendered 

 opaque by immersion in spirit. Each band is again sub- 

 divided, but less distinctly, into a right and left portion ; so 

 that in fact the muscular apparatus might be described as 

 consisting of four bands, a dorsal and an abdominal on either 

 side. These muscular bands are composed of long trans- 

 versely striated fasciculi resembling those of insects. 



The disposition of the muscular apparatus would indicate, 

 as observation shows to be the case, that the movements of the 

 animal are chiefly those of flexion and extension in the trans- 

 verse plane of the body, and consequently that the Sagitta, as 

 was observed by Quoy and Gaimard, swims like a Cetacean 

 by the horizontal blows of its caudal fin upon the water. 



The Nervous system, in Sagitta bipunctata, may be described 

 as consisting of two principal ganglions, one situated on the 

 dorsal aspect of the head, the cephalic ganglion, and the other 

 on the ventral aspect of the trunk, the ventral ganglion. The 

 one consequently is above the oesophagus, and the other below 

 it, — supra and sub-cesophageal ganglia. These ganglia — which, 

 as well as the nervous trunks, lie immediately beneath, and in 

 close contact with the integument — are mainly composed, as in 

 other instances, of ganglionic cells, but in the ventral ganglion 

 there appears to be a certain amount of white nervous matter 

 in the centre (fig. 8, A). 



The cephalic, or supra-oesophageal ganglion (fig. 7), is 

 situated in the mesian line, a short distance from the anterior 



* L. c, p. 6. 



