432 TJie StructJire and Life-History of a Sponge. [Sess. 



silicious particles or spicules ; while in another type, calcareous 

 spicules make np the entire skeleton. Professor Grant was 

 the first to point out this fact, that while the spicules in some 

 sponges were silicious, in others they were composed of car- 

 bonate of lime. We are thus furnished with a natural and 

 convenient grouping of the sponges into four well-marked 

 divisions — viz. : 1st, where the sponge-skeleton is composed 

 of horny fibres, and where spicules are entirely absent, as in 

 the economic sponges ; 2d, where the skeleton is composed 

 extensively of silicious or flinty elements, but where the 

 horny fibres are still present ; 3d, where no horny framework 

 is found, and the skeleton is made up wholly of silicious par- 

 ticles ; and, 4th, where the horny framework is still absent, 

 but the skeleton is wholly composed of calcarcotis spicules. 

 The various shapes of these spicules have also been made 

 use of in classifying the sponges. They are generally very 

 minute, though varying as much in size as they do in shape, 

 reaching their largest dimensions in the now well-known 

 hexactinellid sponge, Euplectella aspergillum, where the 

 spicules, at first free, become ultimately cemented together 

 or vitrified, and form the lovely interlacing network popularly 

 known as " Venus's Flower-basket." 



As spicules are favourite objects with many microscopists, 

 a short description of them may be interesting. They are 

 amongst the earliest developed organs of the sponge, and are 

 " composed of an organic basis (spiculin), densely impregnated 

 or chemically combined with a mineral salt — carbonate of 

 lime in the case of calcareous spicules, silica in that of sili- 

 cious spicules." Over two hundred forms are figured by Dr 

 Bowerbank in his ' Monograph of the British Spongiadte,' 

 grouped under a natural classification — viz., spicules of the 

 skeleton, of the membranes, and of the ovaries and gemmules. 

 To enumerate the diverse shapes of sponge-spicules would be 

 a difficult task. While many are needle-shaped or rod-like, 

 others are variously radiate, hooked, anchorate, globular, 

 branched, and so on in numberless modifications. Those 

 present in the mesoderm are smaller, and show a greater com- 

 plexity of shape, than those which aid either in forming or in 

 strengthening the framework. The vast number of spicules 

 found in some sponges, as already mentioned, is astonish- 



