126 NOTES ON THE ANATOMY OF DINOPHILUS. 



■was extremely difficult, but it may be taken as probable that the ex- 

 ternal aperture, in each case, is at a level between the two rings of 

 cilia possessed by the segment to which a given nephridium belongs. 

 The inner end of the first nephridium is very slightly behind the 

 principal (second) praeoral ring of cilia j this nephridium opens to 

 the exterior on the first body-segment, and may be regarded as the 

 equivalent of the head-kidney of a Trochosphere larva. The second 

 nephridium commences at the anterior end of the stomach, runs at 

 first dorsal to the testis, then bending round to open to the exterior 

 on the ventral surface of the second segment. The third nephridium 

 lies at the level of the middle segment, and, like the second, has its 

 excretory portion situated on the dorsal surface of the testis, its duct 

 curving round to open ventrally on the third segment. The fourth 

 nephridium lies, in the fourth segment, on the ventral surface of the 

 stomach, its internal end occurring close to the aperture from the 

 stomach into the intestine. Its duct, unlike the ducts of the second 

 and third nephridia, runs entirely ventral to the testis. 



The internal end of each of the above nephridia lies in a perfectly 

 definite space, which contains an orange fluid and which is probably 

 merely a specialised portion of the general body-cavity. It is almost 

 certainly the case that the spaces which surround the internal ends 

 of the nephridia are continuous with one another, as shown on the 

 right side of fig. 15. In the case of the first three nephridia, the 

 space in question lies on either side of the alimentary canal, and in 

 living specimens was usually most readily distinguishable in the re- 

 gion of the third nephridium, as a distinct cavity, apparently with- 

 out proper walls, between the stomach and the membrane of the 

 testis. In transverse sections it could usually be seen that this part 

 of the body-cavity extended to the ventral side of the stomach {v. fig. 

 13), whilst in the region of the fourth nephridia, the median portion 

 of the cavity was, in most specimens, observed to pass down ven- 

 trally as far as the skin, thus dividing the testis, in this region, into 

 two symmetrical, right and left lobes. In the median space thus 

 formed are situated the internal ends of the fourth nephridia. 



The remainder of the general body-cavity consists of a meshwork 

 of spaces, filling up the intervals between the various organs and 

 the skin. These spaces are, like those described by Weldon in 

 D. gigas, devoid of an epithelial lining. Many of the cells which 

 bound these lacunae are large, branching connective-tissue cells, 

 which contain an orange pigment. The pigmented cells are usually 

 more numerous in the male than in the female, their pigment in tlie 

 female being often markedly paler in colour than in the male, whilst 

 (in the female) their tint tends to be yellow rather than orange. 

 The difference in the colouration of the two sexes, above alluded to 



