PROTOZOA : SPONGIDA. 6/ 



provided with numerous cilia by which they move about actively, 

 becoming finally attached to some solid body, and developing 

 themselves into the adult sponge. 



In the second or sexual method of reproduction, certain of 

 the sponge-particles or "sarcoids" separate themselves and 

 become nucleolo-nucleated, thus coming to resemble ova. At 

 the same time other sarcoids become motionless, and their 

 contents become molecular, and are finally converted into sper- 

 matozoa. By the rupture of these, and by the consequent 

 contact of the different elements, embryos are produced, 

 which are at first ciliated and move about freely, becoming 

 eventually stationary, and developing into new individuals. 



Classification of the Sponges. — The Spongida have been 

 variously classed, and a good natural arrangement is still a 

 desideratum. By Dr Bowerbank they are somewhat arbi- 

 trarily arranged in three orders — viz., the Keratosa, the Silicea, 

 and the Calcarea, of which the first is believed to hold the 

 lowest place. In the Keratosa the skeleton is composed of 

 interlacing horny fibres, usually strengthened by spicula either 

 of flint or lime. In the Calcarea or Calcispongice the skeleton 

 is composed of carbonate of lime ; whilst in the Siliceous 

 sponges it is composed either of spicules of silex, or " of solid, 

 laminated, and continuous siUceous fibre." By Professor Wy- 

 ville Thomson the siliceous sponges are arranged in a separate 

 order under the name of the " vitreous sponges " ( Vitred). The 

 nature of the skeleton thus varies considerably, whilst tlie spi- 

 cules show almost indefinite modifications of shape, though 

 they are constant for any given species, in any .given part of 

 its organisation. The sponge-flesh is much more uniform in 

 its nature and composition. It may be noticed, however, that 

 in Spongilla the sponge-particles are filled with green granules, 

 which are apparently identical in chemical composition with 

 the green colouring matter of plants {chlorophyll). In Grantia', 

 too, the sarcoids are furnished with long filamentous append- 

 ages or cilia (fig. 2, d). The siliceous sponges are mostly in- 

 habitants of the deep sea, and many of them are remarkable 

 for their long and slender spicules of flint. In Hyalonema, or 

 the glass-rope, long placed amongst the Zoophytes (Zoantharia 

 sclerpbasica), there is a cup-shaped sponge-body, supported by a 

 rope of long twisted siliceous fibres, which are sunk in the mud 

 of the sea-bottom. In other "anchoring sponges," such as 

 Pheronema and Holtenia, the body is sessile or stemless, and is 

 moored to the mud by a beard of long delicate spicules. These 

 sponges, in their single, long, chimney-like osculum, show a 

 curious resemblance to the fossil Siphonias of the greensand. 



