PORIFERA 117 



one family, the Spongillidae, occurs in fresh water, but its members 

 are plentiful and widespread. 



The affinities, and therefore the systematic position, of the phylum 

 Porifera have been the subject of much dispute. In that their bodies 

 consist of many ''cells", they might seem to be metazoa. But they 

 differ from all members of that group in several important respects. 

 In no metazoon are choanocytes found. In none is the principal 

 opening exhalant. In none is there during development an inversion 

 whereby a flagellated outer covering becomes internal. Lastly, and 

 perhaps most significantly, in a sponge the "cells" are far less 

 specialized and dependent upon one another than the cells of a meta- 

 zooQ. Many of them can assume various forms, becoming amoeboid, 

 collared, etc. Many are isolated in the jelly, and when they touch they 

 are often not continuous. No nervous system co-ordinates their 

 activities. Even the choanocytes, though the sum of their efforts 

 produces a current, do not keep time in their working. In short, the 

 Porifera are practically colonies of protozoa. Moreover, it would 

 seem that they took origin from choanoflagellate mastigophora. No\v 

 opinion is, as we have seen, not unanimous that the Metazoa arose as 

 colonies of protozoa, and in any case it is unlikely that they sprang 

 from choanoflagellates. Thus the sponges, in spite of certain super- 

 ficial resemblances to the Metazoa, have no real similarity to, and 

 probably no genetic affinity with, that subkingdom. For this reason 

 it is best that, in a classification of animals, they should be given, 

 under the name of Parazoa, the same rank as the Protozoa and the 

 Metazoa. 



Class CALCAREA 



Sponges with skeletons consisting solely of calcareous spicules ; and 

 with large choanocytes. 



Clathrina. A meshwork of Ascon tubes. The nuclei of the choano- 

 cytes are at the bases of the cells. British. 



Leucosolenia. A clump of erect Ascon tubes, each of which may be 

 branched, connected at their bases. The nuclei of the choanocytes 

 are apical. British. 



Sycon (Fig. 99). A simple vase with a canal system of the second 

 type, having the thimble-shaped outgrowths little adherent to one 

 another. The nuclei of the choanocytes are apical. British. 



Grantia. Differs from Sycon in that the outgrowths which contain 

 the flagellated chambers adhere in many places and are covered by a 

 cortex (Fig. 104). British. 



Leucandra. Canal system of the third type (Fig. 100). Nuclei of 

 choanocytes basal. British. 



