PORIFERA 



127 



(Fig. 58, C). The final stage in this process is exemplified by the ordinary- 

 Toilet sponges 1 in which the spicules have completely vanished, leaving 

 behind them the spongine framework. The Toilet sponge as purchased 

 is simply this skeletal framework from which the protoplasmic tissues 

 have been removed by putrefaction or maceration. 



Amongst the Demospongiae are included the few genera, such as 

 Spongilla and Ephydatia, which have forsaken the sea and live in fresh 

 water. It is more especially these which have developed the power of 

 forming gemmules. - '' 



Zoologists are very generally inclined to regard the Porifera as a group 

 which has arisen in the course of evolution from the Protozoa inde- 

 pendently of the Coelen- 

 terata and other Metazoa. 

 They take this attitude 

 for two reasons amongst 

 others. (i) The early 

 stages in the development 

 of the few types of sponge 

 in which they have been 

 investigated are peculiar 

 and do not provide any 

 evidence to show that 

 the osculum of a simple 

 sponge corresponds to the 

 primitive mouth of the 



COelenterate. (2) Perhaps Proterospmgia. x 600. (From The Cambridge Natural 



the most striking charac- Htsiory— after SaviUe Kent.) a, Amoebocyte ; 6, cell-individual 

 . . T^ ., ■ undergoing fission ; t, cell with small collar ; 2, jelly. 



teristic of the Porifera is 



afforded by the choanocytes— a type of cell which, if it occurs at all, is of 

 the greatest rarity in other Metazoa. Now there is a special section of the 

 Protozoa, known as the Choanoflagellata, in which tlje cell individual is 

 identical with a free-living choanocyte. Further a special genus of Choano- 

 flagellata is known {Proterospongia— Fig. 6i) in which a number of cell 

 individuals remain associated together as a community. Some of these 

 remain typical choanocytes while others, embedded in a jelly-like secretion, 

 become amoebocytes. In Proterospongia we see actually existing a 

 creature which may reasonably be interpreted as representing a first 



1 These belong to the genera Euspongia — the finer-textured sponges, and Hippo- 

 spongia — the coarser sponges, in which the body of the sponge is traversed by 

 numerous wide branching channels in addition to the normal canal system. 



