38 



SPONGID^. 



diate substance. The longitudinal axis of each of 

 the rods is traversed by a very narrow canal. 



To return to the intermediate substance. Its 

 granular mass is found to be altogether made up 

 of small circular cells (a, (o), "and of sperma- 

 tozoa in every stage of development from those 

 cells. The cell throws out a long filament which 

 becomes the tail of the spermatozoon, and becoming 

 longer and pointed forms, itself, the head. The 

 perfect spermatozoa have long, pointed, somewhat 

 triangular heads about -3-0^-0 of an inch in diameter, 

 with truncated bases, from which a very long fili- 

 form tail proceeds." 



" The ova (a, o) are of various sizes. The largest 

 are oval and about 3^^ of an inch in long diameter. 



Fig. 7. 



\ 



^g^^^^ <> 



Structure of Tethta : — a, portion of the intermediate sub- 

 stance of Tethya (magnified), showing 0, ova embedded in sper- 

 matic mass («), together with stellate bodies (k); — b, section of 

 Tethya (nat. size), showing A, central portion, iS, intermediate 

 substance, a, cortical layer, 5, canals. 



They have a very distinct vitellary membrane, 

 which contains an opaque coarsely granular yolk. 



, 55 



