HYDROIDS. 837 



is to say they arise from luithin perfectly normal hydrothecae, placed in a row with the others, 

 but containing no hydranths. What is the meaning of this ? Whether there were ever a 

 hydranth in the hydrotheca or not it is as yet impossible to say, though it is known that 

 hydranths may die and be replaced, but at present the presumption must be that there was, and 

 that its place has been taken by the blastostyle. The question is rather what may be the 

 relation of the latter to the former'. Two things suggest themselves. Either the blastostyle 

 may be an outshoot of the hydranth comparable to the gonophore-stalk of Tubularia, or it 

 may be equivalent to the hydranth, and thus show a normal case of the same process which 

 leads to the occasional replacement of one of the limbs of an arthropod by a limb of a 

 kind found in another part of the body (9). There are, however, some considerations which 

 make the latter of these two cases the more likely. If the blastostyle were an outshoot of 

 the hydranth there would be no more reason for its having a theca than for a tentacle 

 to be so covered. Again, in Thecocladium the same replacement of a hydranth by another 

 member of the body takes place, only here it is a branch that grows out of the hydrotheca 

 (6, II. p. 80). But the strongest witness to the unlikelihood of the gonophore-stalk suggestion 

 is borne by the present new species of Sijnthecium in which, besides the gonothecae which 

 arise from within hydrothecae, there are others growing from the stem between and among 

 them. It is far more likely that these are homologous with the scattered gonothecae of 

 other Campanularians, which we have traced through Ualecium and Lictorella to whole 

 hydranths, than that they represent gonophore-stalks borne formerly on the body of hydranths 

 that have now disappeared, or were perhaps never developed when the stalks became scattered 

 outside the hydrothecae. 



2. Faunistic conclusions are hard to draw fi'om a collection of Hydroids, and that for two 

 reasons — the tendency in the group towards wide distribution, and the scantiness of our 

 knowledge. At the same time there are undoubtedly strong distinctions between the faunas 

 of certain localities, as for instance between that of the north of Australia and either that 

 of the south of the same continent on the one hand, or that of Bi'itain, for instance, on 

 the other. It is therefore possible to discuss the affinities of any given locality to others, 

 even though general conclusions as to the nature and limits of the regions have not as yet 

 been reached. 



No Hydroids were brought back from Minikoi, but the general character of its fauna 

 in that group is, no doubt, well enough shown by the Maldive collection. This contains, 

 as has been said, specimens of 23 species, of which 7 are identical with, or closely related 

 to, North Australian forms, 5 to South Australian, 3 to New British, 3 to West Indian, 

 2 to North-east Atlantic, and 1 to a Cinghalese speciesl Thus the greatest likeness, so 

 far as numbers show, is to the North Australian fauna. At the same time, in view of 

 the very much smaller number of species known from New Britain (eight), the fact that 

 as many as three in the far from large Maldive collection are also found in the former 

 locality is very striking, especially as they have not hitherto been recorded elsewhere. Another 

 interesting feature of the collection is that it shows nearly as much kinship to the fauna of 

 the south of Australia as to that of the north, though these are very distinct from one 

 another (6, 7). The finding of North-east Atlantic and of West Indian forms is not so note- 

 worthy as it would be in a group with less tendency to the wide distribution of its species, 



1 Whether it were ever actually present or not. any already known. Others are recorded from more than 



^ Some of the species are new and not closely akin to one part of the world. 



