152 ZOOLOGY. 



219. The Acanthometra (Gr. acanthos, a thorn; 

 metra, the womb) have a skeleton of flinty spines which 

 radiate from the centre of the gelatinous body. Some of 

 the pseudopodia pass through canals in the interior of the 

 spines ; others proceed directly from the mass of sarcode. 

 They do not form a network. They are smaller than the 

 Thalassicollida, and are found floating at the surface of 

 the sea. 



220. The Actinophrys, or sun-animalcule, is also re- 

 ferred by Dr. Carpenter to this group. 



221. (4.) Spongida. Until recently, the sponges were 

 considered to be vegetables ; but they are now known to 

 be entitled to a place in the animal kingdom. What we 

 call a sponge is merely the framework or skeleton formed 

 by a mass of animals, each of which resembles an 

 Amoeba, and contains a nucleus. These skeletons are 

 generally horny, as in the sponge of commerce ; but 

 there are also calcareous and siliceous sponges. The 

 horny sponges are often strengthened by spicula, or 

 needles of siliceous or calcareous matter. These skeletons 

 are invested by a jelly-like mass, which resembles 

 in appearance the white of an egg. "When the sponge is 

 lifted out of the water, this jelly drains off. This gela- 

 tinous substance is permeated in every direction by canals 

 which communicate with the surface by a great number 

 of small openings, and a few large ones. Corresponding 

 openings may be observed in the fibrous skeleton. Cur- 

 rents of water enter by the smaller or inhalant aper- 

 tures; and, after passing through the system of canals, are 

 driven out through the larger or exhalant apertures. 

 Careful examination has shown that these currents are 

 produced by cilia, which are situated in spherical dila- 

 tations of the canals in the inner layer of the sponge. 

 These cavities are surrounded by particles of sarcode, 

 " each of which is furnished with a vibratile cilium." 

 The cilia all work so as to drive the water out by the 

 exhalant apertures. The currents thus produced bring 

 such particles of food as may be floating in the water, and 



