DEVELOPMENT OF SPONGES 



163 



stage the cells at one pole lose their flagella and become granular, and 

 an amphiblastula results. This invaginates to form a hemispherical 

 gastrula, which settles mouth downwards. Pores, an osculum, and 

 the mesogloea are formed as before, and the inner layer becomes folded 

 into flagellate chambers. 



The main features of sponge embryology are thus summarised by 

 Minchin : — 



" I. The larva is composed of three classes of cell-elements : (i) 

 Columnar flagellated cells, forming the outer covering or localised at the 

 anterior pole ; (2) rounded, more or less amoeboid elements, rarely 

 flagellated, forming the inner mass or aggregated at the posterior pole ; 

 and (3) the archaeocytes, usually scattered in the inner mass, and often 

 represented by undifferentiated blastomeres. . . . 



" II. The larva fixes and undergoes a metamorphosis whereby the 



Fig. 80. — Diagram of early iixed stage of sponge. 



D., Dermal layer of cells ; G., gastric layer of cells ;• 

 BL., cavity of blastula disappearing ; A., archen- 

 teron ; P., attaching processes of the outer layer 

 cells. 



This early stage is somewhat like a thimble, fastened 

 mouth (M.) downwards to the substratum. 



flagellated cells become placed in the interior, while the cells of the 

 inner mass come to surround them completely. 



'■ III. (i) The flagellated cells of the larva become the choanocytes 

 of the adult (gastral layer), acquiring a collar ; . . . (2) the inner mass 

 gives rise to the dermal layer in its entirety : . . . (3) the archaeocytes 

 become the wandering cells of the adult, from which the reproductive 

 cells arise." 



It is interesting to note that the primitive germ-cells are early set 

 apart. 



Classification. — 



Class I. — Calcarea. With skeleton of calcareous spicules : — 



Grade I. — Homoccela. — Continuous internal layer of collared 



flagellate cells, e.g. Ascetta, Leucosolenia. 

 Grade II. — Heterocoela. — Collared flagellate cells restricted to 



radial tubes or chambers, e.g. Sycon (Grantia). 



