124 NOTES ON THE ANATOMY OP DINOPHILUS. 



the arrangement of the ganglion-cells. A similar arrangement is 

 figured by Repiachoff (12^ pi. ii, fig. 10). 



The brain gives off a pair of strong oesophageal commissures (fig. 

 9), which pass round the sides of the mouth to become connected 

 with the ventral cords, as has been described by Weldon in D. gigas. 

 The brain itself is, for the most part, separated from the skin by the 

 basement membrane of the latter. The oesophageal commissures 

 at first lie inside the basement-membrane, but perforate the latter 

 shortly before they become continuous with the ventral cords. 



On the ventral side, in front and on the median side of the origin 

 of the oesophageal commissures, the brain becomes continuous with 

 the ectoderm at two points, one on each side of the middle line (cf. 

 fig. 6). It is probable that the tactile organs of the head itself 

 receive their nerve-supply from this region of the brain, which, how- 

 ever, sends off at the same point an oesophageal nerve (figs. 6, 9, 

 and 10) which may be traced, on each side of the oesophagus, as far 

 as the end of the latter; these nerves were not observed to occur in 

 the proventriculus. The oesophageal nerve supplies the wall of the 

 oesophagus itself, and gives off a branch which can be traced as far 

 as the surface of the muscular appendage. 



The eyes, which are of a bright red colour, lie on the dorsal 

 surface of the brain, immediately below the basement-membrane 

 of the skin (fig. 9). Each consists of a double pigmented sac, 

 filled with a clear substance, which no doubt functions as a lens. 

 In surface view (fig. 1) the cavity of the eye is not seen, but it is 

 shown in the horizontal section, fig. 7. Remembering that the plane 

 of the section, fig. 9, is at right angles to that of the section, fig. 7, 

 the difference between the two eyes in the former is readily accounted 

 for by the obliquity of the section. 



The ventral part of the head is provided with a pair of small 

 sacs, each of which has an extremely fine lumen opening to the 

 exterior at one side of the anterior portion of the mouth (fig. 9). 

 These bodies are presumably sense-organs, since they are supplied 

 by the above-mentioned oesophageal nerves. Similar organs are 

 described by Repiachoff (12, pi. iv, figs. 1, 8, y) in D. gyrociliatus, 

 in which species it must be noticed that they occur in additioia to 

 lateral, cephalic, ciliated pits. 



Body-cavity. — The body-cavity is represented partly by irregular 

 spaces in the loose connective tissue, as described by Weldon in 

 D. gigas, and by Repiachoff in B. gyrociliatus, partly by more definite 

 spaces, which seem to be specially connected with the internal ends 

 of the nephridia. In males which are sexually mature, by far the 

 greater part of the space between the alimentary canal and the skin 

 is taken up by the very largely developed generative organs {v. 



