94 



SPONGES 



type the sponge has more the form of a gi'owth, spreading or compact, 

 without distinct individuals. In the Lcucosolenia type the sponge appears 

 <is a collection of distinct Ulynthits individuals, each throwing out diver- 

 ticula on every side, from which daughter individuals arise by a process 

 of budding. In Clathrina the intercanal system is greatly developed ; in 

 LcMosolenia the term can scarcely with justice be applied to the inter- 

 spaces between the diverticula and oscular tubes. 



In the family Clathrintdae the canal system, though always reducible 

 to the type above described, may undergo certain secondary modifications 

 which may be considered under two heads, according as they affect the 

 gastral cavity or the intercanal system. As an instance of the former 

 kind may be mentioned the frequent widening of the cavity of the 



B 



Fig. 05. 



'", Typi'sof canal system in Ascons. The thick black line reprosents the gastral layer, the 

 ilotte<l line the dermal layer; the pores are not represented. A, Clathrina type; li, Lcuco- 

 soleniu type, osc.t, oscular tube ; t.s, tubar system ; i.c, intercanal system. 



central oscular tube, until it assumes the appearance of a central cloaca 

 or basin, into which the Ascon tubes empty themselves. This modifica- 

 tion has reached its limit in the species Clathrina tripod if era, Carter (type 

 of Bidder's genus Iknih/n), as described by Dondy (1891), in which the 

 tubar system takes on a radiate arrangement round the very large central 

 cloaca. In the genus Ancandra, on the other haiitl, the gastral cavity is 

 divided up by folds of the ga.stral layer, which owe their origin to the 

 great develoiunent of the spicule rays which project from the wall into 

 the gastral cavity. The diverticula .thus formed are not, however, in any 

 way C()mi)arable to those seen in the oscular tube of Lcucosolctiia, since in 

 Ascnudra the folding does not affect the external surface of the body w;dl, 

 but only the gastral layer. 



Modifications of the intercanal .system in the simple Clathrina type 



