124 



THE INVERTEBRATA 



some cases (Spongillidae) the gemmules are remarkable in that they 

 originate as clumps of the amoeboid cells of the parent. They will 

 stand freezing or drought, and carry the species through unfavourable 

 conditions. The power of regeneration and repair is possessed by 

 sponges in a high degree, and they can be propagated artificially by 

 cuttings. 



Sponges are found in all parts and at all depths of the sea. Only 

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

 are plentiful and widespread. 



ost. ih.ch. *'^' 



i ih.ch. 



ih.ch. 



ost. 



spi— 



Fig. 105. Section of a portion of Grantia extusarticulata. Highly magnified. 

 From Dendy. ost. openings of the inhalant canals (ostia) ; ih.ch. inhalant 

 canal ; prp. openings of inhalant canals into flagellated chamber (prosopyles) ; 

 fl.c. flagellated or collar ceils (choanocytes) ; y?.c/z. flagellated chamber; spi. 

 spicules ; ap. exhalant opening (apopyle) of flagellated chamber. 



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 special- 

 ized and dependent upon one another than the cells of a metazoon. 

 Many of them can assume various forms, becoming amoeboid, 



