174 



PORIFERA 



■^ s 



morphosis in sea water which is constantly changed, and will live 

 for some days. 



We have said that the young sponge has only one osculum. 

 This is the only organ which is present in unit number, and it is 

 natural to ask whether perhaps the osculum may not be taken as 

 a mark of the individual; whether the fistular specimens, for 



example, of H. panicea may 

 not be solitary individuals, 

 and the cockscomb and other 

 forms colonies in which the 

 individuals are merged to 

 different degrees. Into the 

 metaphysics of such a view 

 we cannot enter here. We 

 must be content to refer to 

 the views of Huxley and 

 of Spencer on Individuality. 

 But it is advisable to 

 avoid speaking of a multi- 

 osculate sponge as a colony 

 of many individuals, even 

 in the sense in which it is usual to speak of a colony of polyps 

 as formed of individuals. The repetition of oscula is probably 

 to be regarded as an example of the phenomenon of repetition 

 of parts, the almost universal occurrence of which has been 

 emphasised by Bateson.^ Delage^ has shown that when two 

 sponge larvae fixed side by side fuse together, the resulting 

 product has but one osculum. This, though seeming to bear out 

 our point of view, loses weight in this connexion, when it is 

 recalled that two Echinoderm larvae fused together give rise 

 in a later stage to but one individual. 



Fig. 69. — Larva of Gellius varius shortly after 

 fixation. The pigmented pole, originally 

 posterior, is turned towards the reader. li, 

 Marginal membrane with pseudopodia ; x, 

 hinder pole. (After Maas.) 



Ephydatia fluviatilis. 



In the fresh water of our rivers, ponds, and lakes, sponges are 

 represented very commonly by JEpliydatia (Spongilla) fluviatilis, 

 a cosmopolitan species. The search for specimens is most likely 



^ Materials for the Study of Variation, 1894, p. 30. 



2 Arch, de Zool. Exp. (2) x. 1892, pp. 345-498. On the general subject, of 

 adhesion of species, see Bowerbank, Brit. Ass. Rep. 1857, p. 11, who quotes Grant 

 as the first to observe the phenomenon. 



