ajid their RelationshijJ to the Corals. 9 



(social or polyzoic) sponges. Of simple sponges or persons we 

 have examples in Si/cuin and Ute among the calcareous sponges, 

 Caminus among the bark sponges, and EiqylecteUa among the 

 siliceous sponges. On the other hand, Leucosolenia and Nardoa 

 among the calcareous sponges, Eusponfjia among the horny 

 sponges, and SpongiUa among the siliceous forms are com- 

 pound sponges or stocks. 



I do not, like most other authors, regard the characteristic 

 canal-system of the sponges as something quite sj)ecific and 

 peculiar to this class, an arrangement sui generis, but share in 

 the opinion of Leuckart and Miklucho, that it is essentially 

 hoMoiogous with the coelenteric vascular system or gastrovas- 

 cular a})paratus of the corals and Hydromedusas — in fact, of 

 all the Acalephai or nettle-animals. Indeed I am so thoroughly 

 convinced of this homology that I (with Miklucho) designate 

 the largest cavity into which that canal-system is dilated in 

 the sponge-body, and which is usually called the excurrent 

 tube or flue {caminus), as the stomach, or digestive cavity, and 

 its outer orifice, which is usually called the excurrent orifice or 

 osculum, as the buccal orifice or mouth. 



In opposition to this conception two objections especially 

 will be urged — namely, in the first place, that there are sponges 

 with no flue and osculum, and, secondly, that the direction of the 

 flow of water in the sponge-body is not reconcilable with it. 

 As regards the first objection, I think I can invalidate it by a 

 simple reference to developmental history. The sponges with- 

 out flue and without osculum are either primitive sponge-forms, 

 whose ancestors had never attained to the differentiation of this 

 central ])art of the canal-system, or they are retroraorphosed 

 forms whose ancestors have lost stomach and mouth by phy- 

 letic degeneration. The latter stand in the same relation to 

 the more highly developed sponges furnished with mouth and 

 stomach as the Cestode worms to the Trematoda. The Cestoda 

 (in consequence of their stronger adaptation to the parasitic 

 mode of life) have also lost the intestine and mouth, which 

 their trematodiforra ancestors possessed. Most of the mouth- 

 less sponges, such, especially, as the Clistosyca and Cophosyca 

 among the Calcispongite, are probably to be regarded as such 

 retromoii^hosed, and not as originally astomatous forms ; and 

 if their embryos, Avhich are still unknown to us, actually ac- 

 quire a mouth and stomach like the other sponge-embryos, this 

 ontogenetic fact would most decidedly confimi our phylogenetic 

 hypothesis. Sycocystis, the young form of which is provided 

 with a mouth, while the mature form is astomatous, may even 

 now be cited in its favour. 



The physiological conditions of the tcufer-circulation in the 



