RELATIONS OF SAGITTA BIPUNCTATA. 19 



extremity of the head. It is of a more or less quadrangular 

 form and flattened, in large specimens measuring about ^ mm. 

 in length. Three pairs of nervous cords proceed from it. 



1. An anterior (fig. 7, bh) which curves outwards, and then 

 backwards towards the process of the head upon which the 

 buccal hooks are placed, to terminate according to Krobn, 

 in the muscles by which the hooks are moved, close to which, 

 he says, that each nerve presents a minute ganglionic enlarge- 

 ment from which several filaments are given off to be distri- 

 buted to the muscles. 



The posterior pair of nerves arising from the cephalic gang- 

 lion (fig. 7, c c) pass backwards, in a divergent direction, and 

 terminate in a rounded ganglionic mass, in the centre of which 

 the eye (fig. 7, k) is, as it were, imbedded. These optic 

 ganglia, according to Krohn, are composed of distinct ganglionic 

 cells ; but it would appear from Mr. Huxley's observations, 

 that the optic nerves, as they may be termed, also exhibit a 

 smaller ganglionic enlargement immediately before entering 

 the optic ganglion (fig. 7, /). The optic ganglion and the eye 

 lie in a special closed cavity in the integument of the head. 



3. The third pair of nerves arising from the cephalic gang- 

 lion (fig. 7, d d) are given off from the sides of that body, 

 curving backwards and downwards, so as to pass on either side 

 of, and to get beneath the oesophagus, where they approach 

 each other again, and becoming nearly parallel in the mesian 

 line of the trunk, join the ventral ganglion. They constitute, 

 therefore, what may be termed an oesophageal commissure. 



The ventral ganglion (fig. 8) lies in the middle of the ventral 

 surface of the trunk, also immediately beneath the integument, 

 which is seen to be somewhat elevated by it when the animal 

 is viewed on the side. It is situated between the head and 

 the lateral fins, though rather nearer to the latter. It is of an 

 elongated, oval form, and in full-grown individuals about I J 

 mm. long. There may be distinguished in it a lighter-coloured 

 nuclear or medullary substance (fig. 8, h), which occupies a 

 central tract, and a darker-coloured, coarsely granular cortical 

 layer, composed apparently for the most part of ganglion- 

 globules. Four principal nervous trunks proceed from this 

 ganglion : an anterior pair (fig. 8, d d), which are continuous 

 with the lateral trunks given off from the cephalic ganglion 

 (fig. 7, d d), and constitute the oesophageal commissure ; and 

 a posterior, (fig. 8, /"/), which run directly backwards, slightly 

 diverging from each other. These trunks are, upon the whole, 

 stronger and rather shorter than the anterior pair, inasmuch as, 

 according to Krohn, they do not extend much beyond the an- 

 terior pair of lateral fins. He states that each trunk terminates 



c 2 



