THE SLIGHTLY COMPLEX ANIMALS OR SPONGES 35 



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. 



33. Development of the sponge. Like all many-celled 

 animals, the sponge begins its life, however, as a single cell 

 the egg which is in this case barely visible to the sharp 

 unaided eye. Fertilized by its union with a sperm cell, 

 development commences, and the first apparent indication 

 of the process will be the division of the cell into halves 

 (Fig. 14, 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 divi- 

 sions of the cells still continue and result in the formation 

 of a hollow globe of cells (called the Uastula, Fig. 14, 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. 14, 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. 



