478 TEXT-BOOK OF ZOOLOGY 



older and large specimens. A young sponge (as shown in the illustra- 

 tion) invariably possesses only one osculum, and it is only after new 

 sponges, all of which remain attached to it, have been produced from it 

 by lateral budding that we meet with several of such apertures. Hence 

 an old sponge represents a colony which has been produced from a single 

 individual by gemmation. 



(e) The actual body of a sponge is much softer than that of the 

 fresh- water polyp (which see, Section C). It acquires, however, a certain 

 firmness by the deposition in the protoplasm of thousands of hard, 

 pointed needles or spicules. They consist of lime or of silica which has 

 been taken up by the sponge from the water and again separated. The 

 spicules are usually grouped together in bundles, forming a solid skeleton, 

 which may be distinctly seen in dried specimens. 



D. Reproduction. 

 In addition to. multiplying by bud\s (see Section C, d) and eggs, the 

 fresh-water sponges continue their species by another very peculiar 

 mode of reproduction. In the autumn spherical bodies about the size 

 of a mustard-seed, and surrounded by a thick coriaceous capsule, are 

 found in the interior of the sponge. These bodies are known as 

 gemmules. Whilst the sponges are (with few exceptions) killed by the 

 winter cold, these gemmules survive, and give rise in the following spring 

 to new sponges. They are also able to survive the complete drying-up 

 of the water in which they were produced (a- very important fact in hot 

 districts) ; hence they play a part analogous to that of the hard- shelled 

 eggs of fresh-water polyps. (See p. 469. Why do marine sponges not 

 need gemmules for the purpose of their preservation ?) 



Other Sponges. 



The sea, contrary to fresh water, possesses a superabundance of 

 sponges of the most varied form and colour. Most of them are sessile, 

 like fresh-water sponges, and united in colonies, their soft bodies being 

 supported by silicious or calcareous spicules or by horny fibres. Among 

 the few which are important to man, the Bath Sponge {Euspongia 

 officinalis) holds the most prominent position. Its soft and elastic horny 

 skeleton, which exactly reproduces the shape of the living animal, is 

 generally familiar. The animal lives in the Mediterranean, especially in 

 the Adriatic. The sponges are detached by men in boats, by means 

 of long four-pronged forks, and pressed out on shore ; the decaying soft 

 tissues are next removed, and the sponge is finally thoroughly rinsed in 

 fresh water. The less valuable sponges with large circular holes belong 

 to another species, the so-called Horse Sponge (Hippospongina equina), 

 from the north coast of Africa. 



