SPONGES 93 



regions ; first, at the oscular rim, resulting in elongation of the 

 tubular body ; and secondly, at certain spots on the surface of the 

 body, leading to the formation of hollow diverticula or outgrowths 

 of the body wall. The diverticula grow out into tubes which 

 become branched and anastomose with one another, giving rise to 

 a more or less complicated network surrounding a central oscular 

 tube, which represents the original Olynthus (Figs. 2-7). New 

 oscula arise either by the perforation of the blind ends of diverticula 

 growing out from the tubar system in a vertical direction, or by 

 fission of a previously existing oscular tube. In the latter case the 

 oscular tube, or, it may be, the primitive Olijnthus becomes first in- 

 folded on each side in a longitudinal direction, so that the transverse 

 section would have the shape of a figure of eight ; and then, by meet- 

 ing of the folds, two distinct oscular tubes are formed. In many 

 cases the fission of the Ohjnthus or oscular tube may stop short of 

 the osculum, so as to give rise to two tubes opening together by a 

 single oscular aperture, ami a similar process of longitudinal fission 

 may bring about a multiplication of the tubes in any part of the 

 body. In the stalked species of the genus Clathrina, such as CI. 

 blanra or lacunosa (Fig. 8), the tubar system arises chiefly by in- 

 complete fission of the Oh/nthus and of the tubes thus formed, and 

 scarcely at all by the outgrowth and anastomosis of diverticula ; 

 the latter method is, however, the most usual in Clathrinidae, and 

 occurs always in Leucosolenia. 



The full-grown Ascon individual or colony consists of two parts ; 

 a more or less complicated tubar system (t.s), opening by one or more 

 osmlar tubes {osc.t, Fig. 65). The gastral cavity is continued into all 

 the tubes, which are lined everywhere by collar cells, their wall 

 having in all parts the same structure as the primitive Olynthus, 

 from which they arose. Between the tubes spaces are enclosed, 

 which, as is obvious from their development, are really external to 

 the sponge. In these spaces, which have been termed the intcr- 

 canal system (i.c), the water circulates before entering through the 

 pores into the gastral cavity. 



Two distinct varieties of canal system can be recognised in Ascons 

 which are the result of slight modifications in the mode of growth, and 

 correspond to considerable differences in the external form. In the first 

 variety, characteristic of the family C'latlirinidac (Fig. 65, A), the tubar 

 system is greatly developed, and the oscular tubes are comparatively 

 insignificant, acting as mere vents for the ramified network of tubes of 

 which the body is composed. In the .second variety, characteristic of the 

 family Leucosoleniidae, the oscular tubes are large and conspicuous, and 

 quite overshadow the tubar system (Fig. 65, B). The latter appears 

 either as a series of diverticula from the erect oscular tubes, or as a 

 system of narrow tubes uniting them basally like a stolon, and in both 

 cases branching and giving rise to new oscular tubes. In the Clathrina 



