526 ARTHUR DENDY. 



comprehend the points of distinction between them. We then 

 see that they differ to such an extent in their minute anatomy 

 that we cannot include them both in the same genus. 



From the foregoing description it appears that the first 

 exhibits the canalicular and the second the lacunar type of 

 canal system; in Ridleia the inhalant and exhalant channels 

 are canalicular, and the flagellated chambers are provided with 

 special inhalant and exhalant canaliculi, while in Quasillina 

 the inhalant and exhalant channels are for the most part 

 lacunar, and the flagellated chambers open directly into them 

 without the intermediation of narrow canaliculi. At the same 

 time there is some evidence to show that these two types of canal 

 system cannot be sharply denned from one another, and 

 that flagellated chambers with, and others without special 

 canaliculi may coexist in the same Sponge. 



It is noteworthy that in Quasillina the chambers, although 

 usually opening directly into the lacunae of the canal system, 

 are elongated in form like those of Ridleia, and not, as in 

 the Halichondrina, where such a mode of opening is typical, 

 spherical, or subspherical. 



Both genera are remarkable for the development of the 

 fibrous tissue. In Ridleia it is largely developed in the 

 ectosome proper, and in the wall of the oscular tube, being 

 arranged in well-defined layers of longitudinal and circular 

 fibres. In Quasillina, on the other hand, it is almost 

 entirely absent from the ectosome proper, but is well developed 

 in the wall of the oscular tube, where it forms definite annular 

 ridges in which the close-packed fibres (myocytes) have a dis- 

 tinct, wavy outline. 



The mode of occurrence of the fibrous tissue in this and in 

 other genera in which it occurs, indicates that its function is a 

 contractile one, or, in other words, that the fibres are muscular 

 fibres. The annular bands of fibres around the oscular tube 

 of Quasillina are probably to be regarded as sphincter 

 muscles. 



The genus Quasillina has been recognised for a long time 

 as a member of the sub-family Suberitidae. It is, however, 



