CCELENTERATA 



57 



which are free-swimming, ciliated, and called larvae because so 

 different from the adult (Figs. 42 and 42 bis), pass out and 

 eventually become attached and give 

 rise to a new sponge colony. Many 

 sponges are hermaphroditic, i.e. each 

 individual produces both ova and sper- 

 matozoa, but cross fertilization generally 

 takes place, since the male and the 

 female germ-cells do not mature at the 

 same time in the same individual. 



The sponges are all aquatic and for 

 the most part marine, — there are a few ( FlG - 4 °- Sycc '• ?" 



r tire colony, natural size. (After 



fresh-water species. In the ocean they Parker and Hasweii.) 

 occur at all depths, some being found 



in the tide pools along the shore, others in the deep sea, and 

 they have a very wide distribution, for we find them in all the 

 waters of the world. They vary almost as much in color as in 

 shape, some being bright green, others pale green, others yellow, 

 orange, dark violet, or white, and many have a peculiar and 



Flo. 41. Gemmulae of fresh-water sponges. I, Spongilla heber- 

 kii/nm, external view of two gemmulae surrounded by the sponge- 

 spicules; 2, Spongilla fluviatilis, a single gemmule more enlarged, 

 seen in section, showing the thick wall with its radial siliceous 

 supports, the amphidiscs and the single opening closed by a mem- 

 brane; within is the mass of germinal cells. (From a Leuckart- 

 Nitsche wall-chart; permission of Dr. C. Chun.) 



characteristic odor. As we have already noticed, the adult 

 sponge is always attached, and its lack of motion caused the 

 sponges for a long time to be classified as plants. Some 

 sponges, however, live attached to other animals, but not as 

 parasites, for they derive no nourishment from them, but rather 

 in a state of symbiosis, as, for example, where they are found 



