FRESH-WATER SPONGE. 



251 



The males appear to perish, but the females grow, and are stated to lose their 

 oscula and enteric cavities (= ampullae) more or less completely. The ciliated 

 embryo produces an individual which is never sexual, but gives origin in autumn to 

 gemmules. There is therefore an alternation of generations (Marshall). 



The green colour of Spongilla is developed only when the sponge grows freely 

 exposed to light. If it is shaded, a pale flesh-colour takes the place of green. 

 However, a piece of such a pale-coloured sponge turns green when dipped into 

 sulphuric acid, as does the colourless saprophyte plant Neottia. A flesh-coloured 

 sponge contains in its cells angular particles ; these in pale-green specimens are 

 found mixed with green concavo-convex discs, which appear to be derived from 

 them, and which are present in great abundance in an ordinary full-green specimen. 

 These green bodies have been considered by Brandt (op. cit. p. 245 ante) to be 

 symbiotic algae. The green colouring matter itself appears to have a similar consti- 

 tution to that of higher plants, but the proportions of the constituents are different. 

 The cells of Spongilla, whether flesh-coloured or green, contain starch in solution. 



The structure of Spongilla has not been properly worked out, and the anatomy 



4' 



Fig. 12. A. Section of Euspongia officinalis, after Schulze, op. cit. infra, PL xxxvi. Fig. 2. 



B. Ampulla of same (Schulze, PI. xxxvii. Fig. n in part). 



C. Young secondary spongin fibre surrounded by spongoblasts (j/.) of same (Schulze, 



PL xxxvi. Fig. 6 in part). 



of a sponge (Fig. 12, A. B.C.) is best illustrated by that of the sponge of commerce, 

 Euspongia officinalis, which grows in quantity in the Mediterranean Sea. There 

 are, according to Schulze (op. cit. infra], six varieties of it, differing in external 

 shape, disposition of the oscula, and of the fibrous skeleton. 



The living organism is dark violet-grey, passing into yellowish brown where the 

 light does not reach it freely. The surface is beset with minute elevations or 

 conuli (A : c.\ and is marked by fine close-set ridges which divide it into areae. 

 Within these areae are situate the microscopic pores (A: /.) by which the sea-water 

 finds its entrance. The number of oscula or exhalent apertures varies in a given 

 specimen. The pores lead either into the dilated commencement of an afferent 



