l62 



ZOOLOGY. 



the separation of gemmules or groups of cells produced asex- 

 ually within the mesenchyma. These, after a period of rest 

 escape and produce new individuals. Sexual reproduction also 



FIG. 78. 



FIG. 77. Diagrams showing the arrangement of the radiating canals in two types of 

 sponges: A, Ascon type; B, Sycon type; C, a portion Or) of the latter, more highly 

 magnified, showing character of the three layers, ec., ectoderm; ent, entoderm (flat- 

 tened layer); en 2 , flagellate entoderm; e.p., external pores; i.e., flagellate chambers 

 of the radiating canals; i.p., internal pores; mes., mesenchyma; r.c., radiating canals; 

 D, two flagellate cells more highly magnified. After Korschelt and Heider. 



Questions on the figures. Trace the relation of ectoderm to the 

 entoderm in these two types? Compare these with illustrations in refer- 

 ence texts. Is there any way of accounting for this disproportionate 

 growth of the entoderm ? What are the apparent functions of the flagellate, 

 collared epithelium? What structures are to be found in the mesenchyma 

 in sponges? 



FIG. 78. Axinella polypoides, showing numerous oscula. After Schmidt. 



Questions on the figure. What are the principal external differences 

 between Axinella and Leucandra (Fig. 74) ? How many individuals are 

 represented here? What are the grounds for your answer? Compare this 

 with the skeleton of the sponge of commerce. 



