12 



BRANCH PORIFERA 



(sperm cells) and the female elements or eggs (ova) are con- 

 tained in the same individual (Fig. 5). It is from the union 

 of a sperm cell with an ovum that the new individual sponge 

 is developed. The sperm cells and the ova rarely mature at 

 the same time in the same individual. Hence, the ova in the 

 canals of one sponge are fertilized by the spermatozoa of an- 

 other sponge, which are carried to them by the afferent currents 

 of water in the canals, thus insuring cross-fertilization. The 

 eggs are retained in the canals until the blastula stage of their 



f 



Fig. 5. First stages in embryonic development of the pond snail 

 (Lymnceus): a, Egg cell; 6, first cleavage; c, second cleavage; d, third cleav- 

 age; e, after numerous cleavages (Morula); /, blastula (in section); g, 

 gastrula just forming (in section); h, gastrula completed (in section). 

 (After Rabl.) This may be taken as a type of the earliest development of 

 all many celled animals (Metazoa). (From Jordan and Kellogg, " Animal 

 Life," D. Appleton and Co., Publishers.) 



development is reached, then they are set free and pass 

 out at the exhalant opening or osculum. The fresh-water 

 sponges (spongilld) bear small, seed-like bodies called gemmules 

 toward the approach of winter. The parent sponge dies, the 

 gemmules remain dormant until the next spring, when the rising 

 temperature calls them to renewed life. They grow into 

 mature spongilla, bear other gemmules, and thus the life- 

 history of their race is repeated. 



Animal Mind. Sponges have no well-marked nerve-cells, 

 though the simplest elements of both nerve and muscle have been 



