DESOKlPTiON OF El'HYDATlA ULEMEINGIA. 77 



datia bleinbingia there is no spongin on the surface, and 

 the spicule fibres are, at most, provided with a very small 

 amount at the junction of the spicules. 



This difference is explained by the dissimilarity in the con- 

 ditions of life. On the one hand, Ephydatia blembingia 

 lives in a small pool of water which probably dries up for the 

 greater part of the year; while Spongilla moorei, on the 

 other hand, lives at the bottom of Lake Tanganyika. There- 

 fore, the former may be described as an annual, while the 

 latter — so to speak — is a perennial spongilla. If this differ- 

 ence in the conditions of life under which these two sponges 

 live were to have any effect at all, we would naturally expect 

 the spongin part of the skeleton to suffer most. 



(3) The Canal System. — Owing to the presence of gem- 

 mules in all stages of development, the canal system could 

 hardly be in such a condition as to be capable of minute 

 description, for the formation of gemmules is accompanied by 

 the breaking down of the sponge tissue. Besides, we know 

 of no preserving fluid that does not admit of a considerable 

 amount of disassociation of the tissue cells of the Monaxo- 

 nida. Though they be preserved with the greatest care, and 

 with the best reagents known, free cells are found in great 

 abundance in the interior of the sponge tissues. The presence 

 of so many amoeboid cells is conducive to this state of things. 

 Consequently our remarks on the canal system must be meagre 

 at best. 



As has been stated above, the dermal ostia are micro- 

 scopically small but comparatively numerous. They open as 

 usual into the subdermal cavities, which are large and ex- 

 tensive (fig. 2), and which are lined by cells which possess 

 granular nuclei. These in their turn open into the inhalant 

 canals, which are also well developed, but decrease in size 

 towards the surface of fixation of the sponge. The flagellated 

 chambers are small and numerous, lying about in the extremely 

 loose tissues of the sponge. The exhalant canals, though at 

 first of fine calibre, assume comparatively huge proportions. 

 The oscula, however, by which they open to the exterior are 



