74 CCELENTERATA 



later and leads an independent life, but in most of them the buds are 

 indistinguishably joined together so that a colony results, the members 

 of which form a compact whole. 



Gemmule formation occurs in all fresh-water and some marine 

 sponges and is a provision against cold weather in temperate regions 

 and in the tropics against desiccation. With the approach of the unfa- 

 vorable season, archeocytes migrate to one spot in the middle layer, the 

 cells of which secrete a capsule around them. When the sponge dies 

 the gemmule falls to the bottom and remains, in the case of the fresh- 

 water sponge, until the following spring, when the capsule bursts and 

 the archeocytes within move out and develop into a sponge. Many 

 fresh-water sponges live through the winter, however, notwithstanding 

 the formation of gemmules. 



Some sponges are hermaphroditic, others are unisexual. No special- 

 ized sexual organs are present, the ova and spermatozoa developing from 

 the archeocytes. The ciliated larva swims actively about in the water, 

 but finally attaches itself and after a metamorphosis develops into the 

 adult animal. 



Sponges are world-wide in their distribution and, with the exception 

 of the Spongillidae, are all found in the sea, where they range from 

 tide lines to very great depths. The fresh-water sponges occur in lakes 

 and streams in all countries. 



History. Aristotle was acquainted with sponges and knew that 

 they were animals, although he notes their likeness to plants. Through 

 the ages following his times, opinions differed concerning them, some 

 people believing them to be plants, others animals, while many ascribed 

 both an animal and a plant nature to them. Ellis (1765) first described 

 the currents of water which stream into and out of sponges. Lamardk 

 classified them with polyps. Robert Grant (1825) definitely proved 

 their animal nature, showing that water flowed into the sponge through 

 numerous minute pores and out through the oscula, and he correctly 

 inferred that ciliary action caused the flow. Many competent natural- 

 ists, however, still believed them to be plants, and the question was not 

 finally settled until about 1870 or later. H. J. Clark (1866) first demon- 

 strated the collar cells and laid the foundation for the belief which pre- 

 vailed for some time that sponges are colonial flagellate Protozoa. The 

 embryological investigations of F. E. Schulze and others, however, have 

 tended to overthrow this belief, and sponges are at present usually 

 classified under the Metazoa either as a separate phylum or under the 

 Coelenterata. 



The Spongiaria include about 2,500 living and a large number of 

 fossil species, and are grouped in 3 classes. 



