PROTOZOA : SPONGIDA. 57 



'hilum' of the gemmule into the water, and the liberated 

 ovules are developed into new Spongillce. 



Spongilla also appears to reproduce itself in a somewhat 

 analogous 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 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 the skeleton is com- 

 posed of carbonate of lime ; whilst in the Siliceous sponges it 

 is composed either of spicules of silex, or ' of solid, laminated, 

 and continuous siliceous fibre.' The nature of the skeleton 

 thus varies considerably, whilst the spicules 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 appendages or 

 cilia (fig. 2 d). 



DISTRIBUTION OF SPONGES IN SPACE. Sponges are almost 

 exclusively marine, and are of almost universal occurrence* 

 The sponges of commerce are mostly obtained from the 

 Grecian Archipelago and the Bahama Islands. Recently the 

 existence of numerous siliceous sponges at great depths in 

 the ocean has been demonstrated . by Drs. Carpenter and 

 Wyville Thomson. They are associated with numerous Fora- 

 mini/era and with Crinoidea, the whole assemblage bearing a 



