THE SPONGES 21 



In such associations as Volvox an important step has 

 been taken beyond that of Pandorina, for there is a division 

 of the labors of the colony among its various members, 

 some acting as locomotor cells while others are germ-cells. 

 These are now so dependent one upon the other that they 

 are unable to exist after separation from the main com- 

 pany, just as a part of the squirrel is incapable of leading 

 an independent existence. A higher type of organism has 

 thus arisen intermediate between the simple one-celled 

 animals and those of many cells, especially the sponges a 

 relation which is more readily recognized after an examina- 

 tion of the latter. 



23. Development of the sponge. As with all many- 

 celled animals, the sponge begins its existence as an egg, 

 in this case barely visible to the sharp unaided eye. Fer- 

 tilized by its union with a sperm cell, development com- 

 mences, and the first apparent indication of the process 

 will be the division of the cell into two halves (Fig. 11, A, B). 

 Each half redivides into four, these again into eight cells, 

 and this process is repeated, giving the young sponge the 

 general form of Pandorina. The divisions of the cells 

 still continue and result in the formation of a hollow 

 globe of cells (called the Uastula, Fig. 11, E, F) similar 

 to Volvox, and at this point the young larva leaves the 

 parent. 



The next transformation consists in a pushing in of one 

 side of the sphere, just as one might press in the side of a 

 hollow rubber ball. The depression gradually deepens, and 

 finally results in the formation of a two-layered sac known 

 as the gastrula (Fig. 11, G). At this stage of its existence 

 the sponge settles down for life in some suitable spot, by 

 applying the opening of its sac-like body to some foreign 

 object. In assuming the final form a new mouth breaks 

 through what was once the bottom of the sac, canals per- 

 forate the body wall, a skeleton is developed, and the char- 

 acteristic features of the adult are thus attained. 



