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ELEMENTARY ZOOLOGY. 



there are others which move about, like white blood corpuscles, and 

 may have something to do with nutrition; there are at certain 

 seasons still others that form eggs and sperm. These unite, in 

 fertilization, to form an embryo and finally a new mature sponge. 



The entoderm cells, lining the passages, are supplied with flagella, 

 the lashing of which produces the inward currents of water which 

 are so essential to the life of the sponge. These flagellated cells 

 apparently have the power of capturing particles of food in some- 

 what the way Euglena takes its food. 



FIG. 43. Diagram to illustrate the development of such a sponge as Fig. 42. 

 1-5 progressive stages from egg to gastrula, ready to become fixed, a, archenteron; 

 bl, blastopore; ec, ectoderm; en, entoderm; mes, beginnings of mesoderm; s, segmen- 

 tation cavity. 



Questions on the Figure. Is cleavage equal or unequal in this 

 egg? How does the manner of forming mesoderm compare with 

 that in Fig. 21? What changes take place between stage 5 in this 

 figure and the condition in Fig. 42 ? 



185. Reproduction and Development of the Sponge. It has been 

 seen that the great masses of sponge, representing a large colony of 

 individuals, may be derived from one egg by a kind of budding 

 or non-sexual reproduction. These non-sexual buds may or may not 

 separate from the parent. There is also the reproduction by eggs 

 and sperm. These are produced by the same parent. The sperm 



