THE SPONGIDA. 13 



of sarcode or protoplasm, from which long slender pseudopodia, 

 which may unite into reticulations, protrude. This protoplasmic 

 substance contains a sac in which are inclosed cellseform bodies, 

 fat globules, coloured granules or crystals, with more or less pro- 

 toplasm. Very generally, numerous yellow corpuscles, which 

 multiply by fission, are scattered through the superficial proto- 

 plasm. To these parts a skeleton may be added, consisting of 

 spicula (which may be loose, or united into a shell imbedded 

 in the superficial protoplasm), or of rods, which meet in the 

 middle of the sac. The skeleton is usually silicified. The 

 more complex forms consist of aggregations of the simpler, which 

 may inclose " vacuoles " or spaces full of water, as in Sphserozoum 

 (Fig. 3). No sexual process has been observed in any Eadio- 

 larian. 



The siliceous skeletons of some of the Radiolaria are known 

 under the name of Polijcistinese, and, like the skeletons of the 

 Foraminifera, they enter largely into the formation of some 

 strata of the earth's crust. 



IV. THE SPONGIDA. 



Multitudinous forms of sponges exist in both salt and fresh 

 waters. Up to the last few years we were in the same case, with 

 respect to this class, as with the Gregarinida, the Rliizopoda, and 

 the Radiolaria. Some zoologists even have been anxious to rele- 

 gate the sponges to the vegetable kingdom ; but the botanists, who 

 understood their business, refused to have anything to do with 

 the intruders. And the botanists were quite right ; for the 

 discoveries of late years have not left the slightest doubt that 

 the sponges are animal organisms, and animal organisms, too, of 

 a very considerable amount of complexity, if we may regard as 

 complex a structure which results from the building up and 

 massing together of a number of similar parts. 



The great majority of the sponges form a skeleton, which is 

 composed of fibres of a horny texture, strengthened by needles, 

 or spicula, of siliceous, or of calcareous, matter ; and this frame- 

 work is so connected together as to form a kind of fibrous 

 skeleton. This, however, is not the essential part of the animal, 



