THE LIFE OP THE SLIGHTLY COMPLEX ANIMALS 35 



becoming inclosed in a common capsular envelope, and by 

 repeated division and consequent increase in number of 

 cells becoming a new sponge. This is reproduction by 

 " budding." The " buds," or small groups of cells which 

 separate from the parent sponge, are called gemmules. 

 Reproduction in the more complex way occurs as follows : 

 Some of the free amoeboid cells of the mesoderm (the mid- 

 dle one of the three layers of the body wall) become en- 

 larged and spherical in form. These are the egg cells. 

 Other mesodermic cells divide into many small cells, which 

 are oval with a long, tapering, tail-like projection. These 

 cells are active, being able to swim by the lashing of the 

 tapering tail. These are the fertilizing cells. The two 

 kinds of reproductive cells may be formed in one sponge ; 

 if so, they are formed at different times. Or one sponge 

 may produce only egg cells, another only fertilizing or, 

 as they are called, sperm cells. Conjugation takes place 

 between a sperm cell and an egg cell. That is, one of the 

 small active sperm cells finds one of the large, spherical, 

 inactive cells and penetrates into the protoplasm of its 

 body. The two cells fuse and form a single cell, which 

 may be called the fertilized or impregnated egg. This fer- 

 tilized egg, remaining in the body mass of the parent 

 sponge, divides repeatedly, the new cells formed by this 

 division remaining together. The young or embryo sponge 

 finally escapes from the body of the parent sponge, and 

 lives for a short time as an active free-swimming animal. 

 Its body consists of an oval mass of cells, of which those on 

 one side are provided with cilia or swimming hairs. The 

 cells of the body continue to divide and to grow, and the 

 body shape gradually changes. The young sponge finally 

 becomes attached to some rock, the body assumes the typi- 

 cal cylindrical shape, an aperture appears at the free end, 

 and small perforations appear on the surface. The sponge 

 becomes full grown. 



It is unfortunate that most of us do not live on the 



