250 DESCRIPTIONS OF PREPARATIONS. 



exhalent apertures are no longer visible ; and its natural emerald-green 

 colour is lost The surface is rough with protruding bundles of the silici- 

 cous spiculae which make up the skeleton of this sponge. The small 

 inhalent orifices or 'pores,' characteristic of the class Porifera, are not 

 distinguishable : indeed they are of microscopic size. It may be noted 

 that the two stems have come into contact by their sides, and that at the 

 point of contact they have fused or undergone ' concrescence.' Concres- 

 cence of this kind is an exceedingly common phenomenon in Porifera, and 

 is one of the causes of the great variability in form of the species. But it 

 does not occur between sponges of different species. Yellow seed-like 

 bodies, the gemmules (statoblasts), may be seen in the substance of the base 

 of attachment : they are formed at the approach of cold weather in the 

 European, at the approach of the dry season in the Indian, fresh-water 

 sponges. 



There appear to be two fresh-water Sponges in Great Britain Spongilla 

 lacustris and Meyenia (Spongilla) fluviatilis. The former is branched, the latter 

 massive and lobate. The principal distinction between the two rests on the 

 structure of the gemmule or statoblast. These bodies are more or less globular in 

 shape, and possess at some one point a pore which is placed at the bottom of a 

 funnel-shaped depression, through which the inclosed mass of sponge cells makes 

 its exit at the proper season. A delicate membrane, finely reticulate, immediately 

 invests this mass of cells, and protrudes slightly into the pore. Externally to it lie 

 two coats a yellowish chitinoid coat and a ' crust,' both of which are deficient at 

 the pore. The crust varies in thickness. In the two British sponges it consists of 

 a granular cell-like structure, which appears to contain silica. In Spongilla lacustris 

 it lodges curved stout fusiform siliceous spicules, the surfaces of which are beset 

 with stout recurved spines. They are arranged tangentially, and give the outer 

 aspect of the crust an appearance like the lines of a so-called ' engine-turned ' 

 watch-case. In Meyenia fluviatilis the spicules of the crust are birotulate, and are 

 known as amphidiscs. They consist of a shaft terminated at each end by a disc 

 deeply and irregularly denticulate at its margin. The amphidiscs are set parallel 

 to one another, and vertically to the outer surface of the crust. Marshall states 

 that the gemmule is formed thus. In autumn a number of amoeboid wandering 

 cells collect in the inhalent canals or ampullae (infra), pass into the mesodermic 

 tissue, and group themselves round one or two mesodermic cells. A pellicle 

 appears on the surface of the mass. Mesodermic cells next form a capsule round 

 it, and are transformed into the crust and its spicules. The parent sponge then 

 dies away. The cells of the gemmule, at first distinct, gradually swell and form a 

 syncytium, which emits a pseudopodium through the pore. In April or May the 

 mass escapes, remains seated on the empty gemmule case, then floats for a couple 

 of days, and finally comes to rest. A clear ectoderm is distinguishable from a more 

 granular internal mass. An enteric cavity appears, before or after the osculum and 

 inhalent pores, both of which may be absent. The young Spongilla becomes 

 sexual. The sexes are separate. The males have no osculum or enteric cavity (?) ; 

 the females have them. The ovum is fertilised, and the ciliated embryo is set free. 



