SPONGES. 393 



colour of chlorophyll to this organism when it becomes 

 green. The transparent intercellular substance of Spon- 

 gilla has a polymorphism equally great with the fully 

 developed cells. This, however, can only be satisfactorily 

 seen when the new sponge is growing out from the seed- 

 like body, at which time it spreads itself over the glass in 

 a transparent film, charged with contracting vesicles of dif- 

 ferent sizes, and in various degrees of dilatation and con- 

 traction. How this substance is produced so early, it is 

 difficult to conceive, since it seems to come into existence 

 independently of the development of the sponge-ovules, 

 which are seen imbedded in it, and there undergoing 

 their transformation into sponge-cells. The spicula, too, 

 are developed synchronously with the advancing trans- 

 parent border, from little glairy globules about the size of 

 the largest ovules, which send out a linear process on 

 each side, and thus gradually grow into their ultimate 

 forms. The only way of accounting for the early appear- 

 ance of this intercellular-substance is to consider that it is 

 a development from some remnants of the original proto- 

 plasm ; and perhaps possesses also the power of producing 

 new sponge-cells, as we see the protoplasm in Vorticella 

 and the roots of Chara producing new buds, independently 

 of the cell-nucleus. 



" The cells of the investing membrane are characterised 

 by their uniformly granular composition and colourless 

 appearance. They are nucleated, possess the contracting 

 vesicle singly or in plurality, and are spread over the 

 membrane in such numbers, that it seems to be almost 

 entirely composed of them ; while they are of such 

 extreme thinness, and drawn out into such long digitated 

 forms, that they present a foliated arraugemeut, not unlike 

 a compressed layer of multifidous leaves, ever moving and 

 changing their shapes. The apertures are circular or 

 elliptical holes in the investing membrane in the cells. 

 Through these apertures the particles of food are admitted 

 into the cavity of the investing membrane. The Paren- 

 chyma consists of a mass of gelatinous substance, in which 

 are embedded the smooth spicules and ovi-bearing cells, 

 and through which pass the afferent and efferent canals. 

 The ovi-bearing cells do not curst and allow their con- 



