326 



ANIMAL DEVELOPMENT 



Fig. S50. — A simple Cup-Sponge [PolylopJitis] producing 

 External Buds. The curving bands below are bundles of 

 siliceous spicules, bunches of which are also seen projecting 

 from the buds. 



to be regarded as colonies, produced by budding or fission from 

 a primary individual. A simple case of the kind is represented 



in fig. 862, 10, but it often 

 happens that the boundaries 

 between the members of the 

 colony are extremely vague, 

 though possibly each larger 

 aperture [oscuium) present cor- 

 responds to an individual. 



In some kinds of Sponge 

 external buds grow out (fig. 

 850), fall off, and develop into 

 adults. A much more interest- 

 ing phenomenon is presented 

 by the formation of internal 

 buds in certain species. The 

 greenish Freshwater Sponge 

 {Spongilla), common in this 

 country, is a particularly in- 

 structive example, for it shows 

 one way by which the unfavourable winter season can be tided 

 over. The internal buds are here little packets of cells enclosed 



in protective coverings made up of 

 peculiar spicules (fig. 851). They are 

 developed on the approach of winter, 

 the cold being fatal to the adult Sponge, 

 which dies and decomposes. The buds 

 [gemmn/es) then fall into the mud, and 

 there remain dormant till the foUowingf 

 spring, when the little mass of cells 

 present in each of them creeps out of 

 the protective investment through a hole 

 present in one place, and grows into an 

 adult Sponge. 



If by accident a living Sponge gets 

 cut or broken into pieces, each of these 

 is able to repair its injuries, and continue 

 erowth on its own account. Advantage has been taken of this 

 possibility in the case of the Bath Sponge {Euspongid), which can 

 be propagated by means of cuttings almost as if it were a plant. 



Fig, 831. ^Section through Gemmule of 

 Frdsliwater Sponge {Spoiigilla), enlarged. 

 a. Cells which secrete the protective spi- 

 cules (.5); c, enclosed mass of cells; (^, place 

 where protective covering is incomplete. 



