PORIFERA 



121 



spicules, of which there exist many different types (Fig. 103), cha- 

 racteristic of various groups of sponges, while minor differences 

 distinguish those of the species, which are often only separable by 

 this means. A horny substance, spongin, may occur as a cement 

 uniting spicules, as fibres in which spicules are imbedded, or as a 

 fibrous skeleton from which spicules are absent. The sponges in 

 which the skeleton is in the latter condition constitute the horny 



f\ 







far 





por 



OHQ 



fit 



^^ exh c 



Oit 



lO^^ 



CP 



ost 



ost 



r 

 I 



ink c 



ink c 3 4 



Fig. 102. Diagrams of the canal systems of sponges. Partly after Minchin. 

 I, Ascon grade. 2, Sycon grade. 3, Leucon grade. 4, Leucon with small, 

 round flagellated chambers, exh.c. exhalant canal; inh.c. inhalant canal; 

 fl.c. flagellated chamber; osc. osculum; ost. ostium ;^ar. paragaster ; /)or. pore. 



sponges (Keratosa), of which the bath sponge {Euspongia, Fig. 104) 

 is an example. Foreign bodies (sand grains, etc.) are often imbedded 

 in the spongin fibres. In a few cases (Myxospongiae) there is no 

 skeleton. The choanocytes of non-calcareous sponges are always 

 restricted to flagellated chambers. Almost without exception these 

 are arranged as in calcareous sponges of the Leucon type, and in 

 most cases the system is made still more intricate by ramifications of 

 the paragaster, the irregular appearance of numerous oscula, which 



