MANUAL OF ZOOLOGY 



SECT. 



complex Sponges the flagellate collared cells are confined to 

 certain ' rounded dilatations of the canals the flagellate 

 chambers. 



Moreover, in the more complex forms the development of 

 branches from the originally simple Sponge, and the 

 coalescence of neighbouring branches with one another, 

 greatly obscure the essential nature of the Sponge as a 

 colony of zooids similar to the branches of Sycon ; and this 



DP 



SD 



In 



FIG. 37. Vertical section of a fresh-water sponge (Spongilla), showing the arrange- 

 ment of the canal-system. C. ciliated chambers ; DP. dermal pores ; Ex. 

 excurrent canals ; GO. openings of the excurrent canals ; PG. paragastric 

 cavity ; SD. subdermal cavities ; O. osculum. (Modified from Leuckart and 

 Nitsche's diagrams.) 



effect is increased by the development of a variety of 

 infoldings of the ectoderm which appear in the higher 

 forms. 



The elements of the skeleton differ in character in the two 

 sub-classes into which the Sponges are divided. In the 

 Calcarea, of which Sycon is an example, they consist of 

 calcareous spicules, usually triradiate in form. In the Non- 

 Calcarea the skeleton either consists of spongin fibres alone 



