98 



COLLEGE ZOOLOGY 



ments by three vertical divisions into a pyramidal plate of eight 

 cells (b, c). A horizontal division now cuts off a small cell from 

 the top of each of the eight, the result being a layer of eight large 

 cells crowned by a layer of eight small cells. The cells now be- 

 come arranged about a central cavity, producing a blastula-like 

 sphere (d) . The small cells multiply rapidly and develop flagelia, 

 while the large cells become granular. The small cells are now 

 partially grown over by the others, forming a structure called the 

 amphiblastula (e). The mass of cells then becomes disc-shaped 

 by the pushing in of the flagellated cells (f). Two layers are 

 thus formed between which the jelly-like middle layer arises. 

 The invaginated side soon becomes attached (g), and the embryo 

 lengthens into a cylinder, at the distal end of which an opening, 

 the osculum, appears (h). In the meantime, spicules arise in 

 the body-wall. 



3. The Fresh- water Sponge — Spongilla 



The fresh-water sponge lives in ponds and streams and may 



be found attached to the under surface of rocks, dead leaves, or 



sticks. It forms incrustations a fraction 

 of an inch thick or compact masses, and 

 is gray or green in color. The structure 

 of Spongilla is shown in Figure 59, C. 

 The canal system is more complicated 

 than that of either Leucosolenia or 

 Grantia. The choanocytes are restricted 

 F e si, m a t° flagellated chambers (C). This is the 



single gemmule, seen in rhagon type, and there are three distinct 



parts to this system: (1) the water passes 

 through the dermal ostia {DP), and, by 

 way of incurrent canals (IN), reaches (2) 

 a number of small chambers (C) lined 

 with choanocytes, thence it is carried 



through (3) an excurrent canal (Ex) to the gastral cavity (PG), 



and finally out of the osculum (0). 



A 



in 



section, showing the thick 

 wall with its opening, 

 and the central mass of 

 germinal cells. (From 

 Weysse, after a Leuckart- 

 Nitsche wall-chart.) 



