CHAPTER III 

 POKIFERA 



Classification adopted 



(Homocoela ( = Asconidae) 



| o 1 n Q y A o F ^ ' 



(Heterocoela fyconidae 



( Leuconidae 



Triaxonia ( = Hexactinellida) 

 Demospongiae 



THE group Porifera or Sponges stand apart from all the rest of the 

 Metazoa, and their embryology is consequently of very great interest. 

 We may suggest as a form for practical study the development of the 

 Calcareous sponge Grantia compressa. 



This sponge, distinguished by its flattened shape, is a common 

 denizen of the British coasts, and its embryology is being worked 

 out by Professor A. Dendy. Allied species occur on the coast of 

 North America ; and the course of its development, so far as deter- 

 mined, so closely resembles that of the Mediterranean species, Sycandra 

 raphanus the subject of Schulze's classic research (1875) that the 

 latter may be taken to represent that of Grantia and of Calcarea 

 generally. 



SYCANDRA RAPHANUS 



The eggs are found embedded in the jelly which forms the sub- 

 stance of the wall of the sponge, intervening as it does between the 

 cells forming the dermal membrane and those lining the paragaster 

 and the extensions of this latter cavity into the flagellate chambers. 

 The spermatozoa occupy a corresponding position in the male. When 

 ripe, they bore their way through into the flagellate chambers, and 

 are discharged by the osculum. They swarm in the surrounding 

 water, and, coming under the influence of the inhalent currents of 

 the female, they penetrate through its pores and thence find their 

 way to the eggs, which are thus fertilized in situ. 



The fertilized egg undergoes the first stages of its development in 

 the maternal tissues. It is found to be contained in a cavity Lined 



37 



