48 Sponges. 



Zoophytes, Order Radiata; Grant's name, however, Porifera, 

 derived from their structure, is that generally received. 



Sponges are destitute of all organs ; they consist of a gelatin- 

 ous substance (sarcode) covering a skeleton, differently composed 

 in the three great divisions of this order. These orders are, i., 

 Calcarea; skeleton, chiefly carbonate of lime : ii., Silicea; siliceous 

 substance forming the skeleton: iii., Keratosa ; named from the 

 peculiar substance Keratode ; which is so called from its possess- 

 ing some qualities (such as elasticity and durability) similar to 

 those of horn, though there is really no affinity between the two 

 substances. All sponges contain sharp and crystalloid spicula. 

 In some cases these form the whole skeleton. Their forms how- 

 ever are never those of real crystals. The sand which we find 

 in new sponges is not natural, but introduced by the natives to 

 increase the weight and value of the sponge. 



The distribution of sponges is universal ; even in fresh water 

 we find the Spongillidae. The sponges of commerce come from 

 the eastern Mediterranean. The Pacific ocean is specially prolific 

 of this order. The continually inci'easing number of British 

 species now, I believe, amounts to about two hundred. 



Some sponges consist of a gelatinous covering, thin as paper, 

 on rocks and seaweed, whence the popular name of " sea flesh ;" 

 while another species, the beautifiil Hyalonema Sieboldi, consists 

 of an ordinary skeleton, from the top of which rises a twisted coil 

 of long siliceous spicula, like the most beautiful spun-glass. 

 Curiously enough, round this coil we always find a band of 

 zoophytes, invariably of the same genus. Some have thought the 

 whole animal to be a fabrication of the Japanese; others (Dr. 

 Bowerbank) think that the band consists of oscula. Another view 

 is, that the spicula and zoophytes are parasitical. This, however, 

 only seems true with regard to the zoophytes. Other species 

 spread themselves by spicula over seaweed, to form an artificial 

 skeleton ; others are shaped like huge trumpets. None, however, 

 can compare with the exquisite " Venus' flower-basket," once so 

 rare, but now costing as many shillings as it once did pounds. In 

 the tropical oceans, the general tendency is to ramification, in 

 colder climates, to condensation. 



Though the sponge is so inconspicuous, on examining it 

 more minutely, we shall find much to rival the organs of more 

 highly developed and conspicuous animals. So first let us turn 

 to the 



Spicula. These are formed of carbonate of lime or silica ; 

 in many cases the presence of keratode unites in an admirable 

 degree, strength, elasticity, and durability. Even in the same 

 sponge, however, we find that some spicula (those required to 



