96 SPONGES 



The modifications of the canal system in the Leucosoleniidae are such 

 as are the direct result of the modifications of the external form which 

 have already been described. It has been shown that the sponge 

 may take on a bushy, arborescent, or creeping form (Figs. 3, 4, and 

 5). Since the canal system follows the external form in its arrange- 

 ment, and is therefore easily understood by simple inspection of the 

 sponge colony, it need not be further considered here. 



(b) The Canal System oftlie Heterocoela. In the calcareous sponges 

 characterised by a discontinuous distribution of the gastral layer 

 and its restriction to the flagellated chambers, the canal system 

 may be of the second or third type, i.e. without or with a 

 system of excurrent canals interpolated between the chambers and 

 the gastral cavity (see above, p. 32). The sub-order Heterocoela 

 comprises all the forms which were classified by Haeckel under 

 the two families Sycons and Leucons, the former having a 

 canal system of the second, the latter of the third type. The 

 grouping of the genera of Heterocoela by characters of the canal 

 system hardly corresponds with their natural affinities, but it 

 is convenient to consider the canal system under its two grades, 

 which we may term the syconoid and leuconoid types respectively. 

 The best examples of the former are seen in the genus Sycon, and 

 of the latter in the genus Leucandra. 



The simplest syconoid type arises from the Olynthus by the 

 formation of hollow diverticula of the gastral cavity, just as in 

 Leucosolenia. The transitory homocoelous condition represented by 

 the young sponge at this stage is, however, soon passed over. 

 Ingrowths of the dermal layer into the gastral cavity take place 

 between the diverticula (Maas, 1898), and as a result of this invasion, 

 comparable to the similar ingrowths which in Ascons form the endo- 

 gastral networks frequently present (see above, p. 48), the gastral 

 layer becomes broken up and discontinuous, and confined to the 

 diverticula or radial tubes, while the general gastral cavity becomes 

 lined by a flat epithelium derived from the ingrowing dermal layer. 

 The sponge has now reached the heterocoelous grade of structure, 

 but even in the adult the upper portion of the oscular tube is often 

 found lined by a continuous layer of collar cells which extend from 

 the uppermost ciliated chamber to the commencement of the oscular 

 rim, and represent a remnant of the primitively continuous gastral 

 layer of the Olynthus. The ciliated chambers have received in 

 Sycons the special name of radial tubes, and they differ further from 

 the diverticula of Leucosolenia in that they remain relatively short, 

 soon attaining their limit of growth, while those of Leucosolenia, as 

 we have seen, continue their growth indefinitely and ultimately 

 give rise to new oscula. Between the radial tubes spaces are 

 enclosed on the exterior of the sponge which are perfectly com- 

 parable in every way to the intercanal system of Ascons, but 



