PROTOZOA : SPONGIDA. 



73 



In the first or asexual method of reproduction, which takes 

 place in the winter, the deeper portions of the sponge are 

 found to be filled with small seed-like rounded bodies, termed 

 " gemmules " or " spores/' each of which possesses a small 

 aperture or "hilum" at one point (fig. 13, //). Eachgemmule 

 is composed of an outer coriaceous capsule surrounded by a 

 layer of peculiar asteroid spicula, resembling two toothed 

 wheels united by an axle, and termed "amphidiscs" (fig. 13, 

 , c). These amphidiscs are embedded in sarcode, whilst their 

 inner surfaces rest upon the tesselated capsule already men- 

 tioned. In the interior of the capsule thus formed is a mass 

 of cells, of which the central ones contain numerous germs. 

 When the spring comes, these masses of " ovi-bearing cells " 

 are discharged through the "hilum" of the gemmule into the 

 water, and are developed into new Spongillce. 



Spongilla also appears to reproduce itself in a somewhat ana- 

 logous manner by means of what are termed " swarm-spores." 

 These are small bodies, containing reproductive germs, and 

 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 

 spermatozoa. 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 Keralosa, 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, often strengthened by spicula either 

 of flint or lime. In the Calcarea or Caldspongia the skeleton is 

 composed of carbonate of lime ; whilst in the Siliceous sponges 

 it is composed either of spicules of silex, or " of solid, lami- 

 nated, and continuous siliceous fibre/' By Professor Wyville 

 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 the 

 spicules show almost indefinite modifications of shape, though 



