276 



GENERAL ZOOLOGY 



When the embryo has reached the blastula stage it is set free, 

 and swims out through the osculum. The larva, in SpongiVla, 

 another fresh-water sponge, and probably in Heteromeyenia 

 as well, is shaped like a hen's egg, but is very small and is 

 covered with collar-cells bearing flagella, which are the organs 

 of locomotion for the few hours of free life Avhich the larva 

 has. When it comes to rest it attaches itself by the broad 

 end, and grows into the form of the adult. 



Fresh-water sponges reproduce also by means of special 

 organs called gemmules, which are about one-fiftieth of an inch 

 in diameter (Figs. 136 and 138). These structures are pro- 

 duced in the sponge-mass toward the end of August. Their 

 use is to carry the species over the unfavorable season after the 

 destruction of the parent sponge, which usually takes place in 

 the fall. They are admirably protected by the three layers of 

 material which form the horny covering. The double-headed 

 spicules give stiffness to the whole gemmule. Inside is a mass 



of cells containing food-material; 

 these cells remain in a state of 

 inactivity until the return of 

 favorable conditions. Then the 

 contents of the gemmule forces 

 itself through the thinly covered 

 pore, and soon forms a minute 

 sponge with incurrent canals 

 and a single osculum, which, as 

 Professor Potts says, " puffs out 

 material after the manner of a 

 miniature volcano." 

 By far the greater number of species of marine sponges, 

 and all fresh-water sponges, are composite in character. In 

 hydroids it is easy to distinguish one individual of a colony 

 from another, but in fresh-water sponges the only clew one 

 can get to the number of individuals is that furnished by the 





FIG. 138. Gemmule of Fresh- 

 Water Sponge. Much enlarged. 

 (After Evans) 



