8 



copic cells each having one lash. The cells are arranged in one 

 layer on the walls of the chamber, and rapidly lashing the water 

 in one direction, they are the cause of this constant stream of water. 

 The inward channel terminates on one side of these chambers, and 

 the outward channels begin on the other side, so the water is 

 sucked in at the pores and driven out violently at the little 

 mouths. The entering currents contain minute particles of 

 food, and also Oxygen in solution, the outward stream contains 

 Carbonic Acid and compounds of Nitrogen and Carbon and 

 Hydrogen, such as are formed by all living animals. The cells 

 will bear a still closer examination, they consist of a spherical body 

 of protoplasm, granular within but firmer and clearer outside, 

 containing a nucleus, and one or more contractile vesicles ; one 

 end rests against the wall of the chamber, and the other, the free 

 end, is prolonged into its cavity with a long clear neck ; around 

 the margin of the free end there is an extremely thin collar, and 

 from its centre a long slender lash protrudes. The collar is a soft 

 semi-liquid material which is constantly circulating, streaming up 

 on the outside and down on the inside, like an endless band in 

 machinery. Particles of food drawn to it by the lashing movement 

 are carried up the outside and down the inside, till they reach the 

 base, where they enter the substance of the cell ; a little drop of 

 water is included with them, and thus the cells not only eat but 

 drink ; the constant circulation of the collar exposes a continual 

 change of surface to the water by which Oxygen is absorbed, and 

 so the cell breathes. 



The tissues of a sponge consist of three layers, an outer, middle, and 

 inner layer of cells. The outer consists of flattened cells with 

 many angles, which cover the whole outside of the Sponge, 

 and line the inward channel. The inner layer lines all the out- 

 ward channels and the lash chambers, and is of the same nature 

 as the outer layer, except in the lash chambers, where, as we have 

 seen already, it consist of the remarkable lash cells. The middle 

 layer is a sort of jelly with stellate angular bodies in it. The 

 skeleton, which we have here, is formed by the cells of the middle 

 layer, and it consists of a network of fibres of spongin, which both 

 chemically and microscopically resemble silk, both substances 

 being regarded as horny matter by chemists. 



The eggs of the Sponge are also found in the middle layer. They 

 are round cells with a large nucleus and nucleolus. In develop- 

 ment these eggs divide first into 2 cells, then 4, then 8, and so on 

 until a globular cluster of similar cells is formed called a morula, 

 from its resemblance to a mulberry. The outer cells now develope 

 cilia or lashes ; and in this state the young sponge generally escapes 

 into one of the outward channels, and is washed away into the 



