104 COLLEGE ZOOLOGY 



from ameboid wandering cells in the middle layer. A ciliated 

 'arva is produced from a holoblastic egg. This larva swims 

 about for a while, thus effecting the dispersal 

 of the species, then becomes fixed and passes 

 through many changes, finally developing 

 ostia and an osculum which are necessary 

 for the nutritive processes and growth. 



One very important peculiarity in sponge 

 embryology is this (Fig. 64): the flagellated 

 cells of the larva do not become the outer 

 Section (dermal) epithelium as do the flagellated cells 

 of the larva of a of the larval ccelenterate (planula, Fig. 73, C, 



but produce the gastral layer of 

 terior granular cells, choanocytes; and the inner cells do not be- 

 come the inner (gastral) epithelium, as do 

 the similarly situated cells in the ccelen- 

 terate planula, but produce the dermal layer. This is shown 

 in Table IV. 



TABLE IV 



THE DEVELOPMENT OF A SPONGE (CLATHRINA) 



Flagellated cells . . . Gastral layer 

 Ameboid inner cells . . Dermal layer 



Ovum-Blastomeres 



Posterior granular cells j Wandering cells 



(Fig. 64, p.g.c.) \ Sexual cells 



It therefore seems impossible to homologize the ectoderm and 

 entoderm of ccelenterates and other METAZOA with the layers in 

 the sponge larva, since the outer layer (ectoderm?) of the latter 

 becomes the inner layer (entoderm?) of the adult sponge. The 

 outer layer is consequently termed " dermal epithelium " instead 

 of " ectoderm," and the inner, the " gastral epithelium " instead 

 of " entoderm." 



(4) Classification. - - PORIFERA. - - SPONGES. - - Diploblastic, 

 radially symmetrical animals; number of antimeres variable; 

 body- wall permeated by many pores, and usually supported by a 

 skeleton of spicules or spongin. 



