STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 165 



given above are^ I believe^ the principal ones to which we 

 must look for an explanation of the peculiarities in the canal 

 system about to be described in different genera of Heterocoela. 



The various genera (with the possible exception ofLeucas- 

 cus) might, as regards the canal system, be arranged in a 

 gradually ascending series, commencing with Sycetta and 

 ending with those genera, such as Leucandra, in which 

 the Leuconoid type reaches its maximum development. Such 

 a series would not, however, as I believe, represent a natural 

 arrangement of the group, for there is, as I hope to show later 

 on, strong reason for concluding that the most highly modified 

 Leuconoid type of canal system has been independently arrived 

 at in several distinct genera. At present, however, we need 

 not concern ourselves with this question, but pass on to 

 consider to what extent the various causes indicated in the 

 above scheme have operated in modifying the canal system 

 in each separate genus. I propose to deal with the genera in 

 the order in which they occur in my system of classification. 

 In this manner, having first of all disposed of Leucascus, we 

 shall be able to start with Sycetta, and trace the gradual 

 evolution of the canal system from its simplest Syconoid form 

 in that genus, to its most complex Leuconoid one as exhibited 

 in Leucandra. We shall then, in each of the two remaining 

 families (Heteropidse and Araphoriscidse) which are dis- 

 tinguished by skeletal peculiarities, be able to start fresh with 

 a Syconoid type of canal system, and work up again to the 

 Leuconoid. 



Leucascus (fig. 1). 



In this genus the sponge is more or less massive or lobate, 

 and it is not possible to distinguish a single central gastral 

 cavity. Tliere may be several oscula, which perhaps indicates 

 that the whole sponge is then to be regarded as a colony com- 

 posed of several fused individuals. The arrangement of the 

 canal system in L. simplex, as seen in a vertical section 

 through the osculura, is shown in fig. 1. The flagellated 

 chambers are very long, and copiously branched. Their blind 

 distal extremities lie beneath the dermal surface, towards 



