170 



ZOOLOGY 



213. Development. Fertilization of the ovum and early 

 cleavage take place in the mesenchyma near the incurrent 



FIG. 80. 



FIG. 79. Diagrams showing the arrangement of the radiating canals in two types of sponges: 

 A, Ascon type; B, Sycon type; C, a portion (*) of the latter, more highly magnified, showing char- 

 acter of the three layers, ec., ectoderm; en 1 entoderm (flattened layer); en 2 , flagellate entoderm; 

 e.p., external pores; f.c., 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 reference 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? 



FlG. 80. Axinella polypoides, showing numerous oscula. After Schmidt. 



Questions on the figure. What are the principal external differences between 

 Axinella and Leucandra (Fig. 76)? How many individuals are represented here? 

 What are the grounds for your answer? Compare this with the skeleton of the 

 sponge of commerce. 



canals, by means of which the spermatozoa find entrance. 

 Cleavage is total and for the most part equal (see 53), pro- 



