AND RELATED GENERA, SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 59 



The arrangement, however, is difficult to determine in detail in Ilyanthopsis, as the 

 grouping of the mesenteries is not known, and only one specimen has been at my 

 disposal, which, moreover, showed an irregular grouping of the tentacles in certain parts. 

 Starting from the one directive tentacle (that where the directive line passes through 

 an oesophageal groove), the arrangement seems to be the following (l, 2, 3, etc., tentacles 

 of the first, second, third, etc., order ; dt. =the directive tentacle) : — 



1 (dt.)-2-l-5, 4-5-4-5-3-4-3-5-4-5-4-5-1-2-1-5-4-5-4-5-4-5-1-2-1-5-4-5- 

 4-5-4-5-1-2-1-5-4-5-4-5-4-5-1-2-1- (dt.)-2-l-5-4-5-4-5-4-5-l-2-l-5-4-5-4- 

 5-4-5-4-5-1-2-1-5-4-5-4-5-4-5-1-2-1-5-4-5-4-5-4-5-1-2. 



It is to be noted here, that among the last mesenteries of the first order, nearest to 

 the last mesentery of the first order, there may be a few tentacles more. A small part 

 of the edge of the body, namely, had been torn away, and it is possible that some 

 tentacles, at most 4 probably, have gone with it. In this specimen, therefore, the 

 number of tentacles is somewhat higher, yet not so high as 100. The group 3, 4, 3, is 

 also somewhat uncertain, in so far as it cannot be determined whether it is a 1 , 2, 1 

 group or not ; in any case it seems to stand somewhat inside the other groups of the 

 first and second order. But to clear up the arrangement of the tentacles definitely 

 requires an investigation of more material with the developmental forms. 



The oral disc is somewhat broader than the base, with deep, radial furrows corre- 

 sponding to the insertions of the mesenteries. The mouth was partly projecting, wide. 

 The oesophagus is wide, long, almost half the length of the body, with a siphonoglyphe. 

 Wassilieff states that two grooves are present, but I have not been able to find more 

 than one ; the other supposed to be present by Wassilieff does not differ — at least in 

 the specimen examined by me, which has also been investigated by Wassilieff — from 

 the deep longitudinal furrows with which the oesophagus is also provided. Including 

 the siphonoglyphe, the number of these furrows amounts to about 16. No well-marked 

 prolongation of the groove downwards below the oesophagus is present. 



The anatomical structure resembles that of the Endocoelactidse. The ectoderm of 

 the body-wall is not extensive, especially by comparison with the thick mesogloea. It 

 contains very numerous spirocysts of varying size, and fairly numerous thick-walled 

 nematocysts (length 34-43 m, breadth 5 n). In transverse sections at the base of the 

 ectoderm we find structures packed close together, which greatly resemble cross-sections 

 of ectodermal, longitudinal muscles. In the beginning also I was inclined to take 

 them as such, but a closer examination showed that this was hardly the case. In 

 longitudinal sections (fig. 2, PI. IV.) I found similar structures, which should not have 

 been there if it was a question of longitudinal muscles. The muscle-like parts are, 

 indeed, so far as I could determine, no other than the greatly thickened bases of 

 ectoderm cells, which can also be clearly seen when the section cuts through a wall of 

 the ectoderm and passes through the ectoderm cells a little way from the base. In 

 these it can be seen that the ectoderm cells are fixed in the mesogloea by greatly 

 thickened bases. I lay stress on this, as M'Murrich was of the opinion that in 



