158 



HA RD WICKE ' S S CIENCE- G OS SIP, 



possibly the parent form of the others ; it is a little 

 mass of protoplasm, containing a nucleus and 

 nucleolus, and closely resembles an ordinary amoeba, 

 or nucleated blood-corpuscle, thrusting out pseudo- 

 podia in various directions during life, and moving 

 freely by their means through the surrounding medium. 

 The innermost layer, or endoderm, forms a con- 

 tinuous lining to the inner wall of the stomach and 

 radial tubes, meeting at the edges of the pores and 

 mouth, the outer layer of plate-like epithelium or 

 ectoderm. It consists, except over the stomach, where 

 it has the same structure as the ectoderm, of more or 

 less flask-shaped cells, each having a nearly spherical 

 body, flattened below, where it is seated on the inner 

 face of the mesoderm, and rounded above and pro- 

 longed into a long neck or colhim. Near its end the 

 collum is surrounded by a delicate collar of transparent 

 sarcode, which gives it the form of a wine-glass. 

 From the end of the collum is produced a long filament 

 of hyaline protoplasm, known as the flagellum, be- 

 cause it flagellates the surrounding water. 



The cell, in general, consists of a more fluid 

 granular central protoplasm, or cndosarc, surrounded 

 by an outer firmer transparent contractile layer, or 

 ectosarc. It is the ectosarc which is extended to form 

 the collar and flagellum, which we may regard as 

 highly specialised derivatives of pseudopodia. In the 

 endosarc is a conical nucleus, surrounded by numer- 

 ous granules, and some little blebs of watery fluid, 

 known as vacuoles. Altogether the cell closely 

 resembles some forms of infusoria, and it is of such 

 cells, arranged close together, side by side, that the 

 endoderm of the radial tubes is wholly composed. 



Before describing the remaining tissues of the 

 sponge, it will be convenient to introduce here a 

 short account of its physiology, so far as the know- 

 ledge we have now attained of its structure makes 

 it possible. 



The flagella of the endodermic cells, which, as we 

 have already noticed, form a continuous lining to the 

 inner wall of the radial tubes, are almost always in 

 motion, bending downwards with a rapid movement 

 in one direction and then returning to their position 

 of rest, and doing this so rapidly that the eye cannot 

 follow them in the active state, so that usually they 

 are quite invisible. Each movement of the flagellum 

 " flicks " the water, as it were, in one direction, and 

 the rapid successive movements of the almost infinitely 

 numerous flagella drive the water out of the radial 

 tubes into the stomach of the sponge, from which it 

 emerges, as we have already stated, in a powerful 

 stream through the mouth. But as water is driven 

 out of the radial tubes the pressure within them is 

 diminished, and to restore equilibrium, — to supply 

 the place of the water which has been expelled, — 

 fresh water flows in from the interior through the 

 pores at the ends of the tubes, and also, after passing 

 down the inter-canals, through the pores at the sides 

 of the tubes. 



The circulation thus established from the pores- 

 through the radial tubes into the stomach, and so out 

 through the mouth, is the means through which, as 

 we shall now see, the nourishment and respiration of 

 the sponge are carried on. 



The water in which the sponge lives is inhabited 

 by a large number of infusoria and other minute forms 

 of life, and contains, besides, many small particles 

 derived from decaying organisms ; these enter the 

 sponge, borne along with the inflowing currents of 

 water, and are seized upon by the flagellate cells of 

 the endoderm, as they pass through the radial tubes. 



The manner in which the flagellated cells extract 

 their food from the water is worth noticing ; it is pre- 

 cisely similar to that in which the flagellated monads, 

 which so closely resemble these cells, feed. No- 

 sooner does a little particle of food touch the edge of 

 the delicate collar which surrounds the collum, than 

 it adheres to it and is carried down by currents that 

 circulate up one side of the collar and down the other 

 to the end of the collum, in which, along with an 

 accompanying drop of water, it becomes at once 

 engulfed. If the particle should come directly in 

 contact with the collum itself, it is engulfed in the 

 same way. The included drop of water, enclosing its 

 particle of food, travels down the collum into the 

 base of the cell, where it forms a little "bleb," which 

 we have already noticed as a " vacuole." The food of 

 the vacuole undergoes digestion, and when all the 

 "goodness" has been got out of it, the indigestible 

 residue is extruded from the cell, through an extem- 

 porised aperture, to be forthwith swept away in the 

 torrent of the circulation, through the stomach, and 

 then out by the mouth. In this way each flagellated 

 cell eats and drinks, living'to itself It also breathes, 

 the water which conveys its food containing dissolved 

 oxygen, which passes into the cell by osmosis. Thus 

 with food for fuel, and oxygen to burn it, the cell is 

 provided with energy, which it expends in maintain- 

 ing the water circulation, from which it obtains food 

 and oxygen again. Though each cell lives its own 

 life, yet the different cells all work more or less in 

 unison ; thus when they have taken enough food to 

 satisfy their wants for the time, they frequently rest to- 

 gether to digest it ; more or fewer of them cease to 

 lash the water, the ostia and pores are closed by the 

 contractile sphincters, which also seem to be in sym- 

 pathy, and tlie circulation goes on feebly, or, for a 

 time, altogether stops, to begin afresh when digestion 

 is completed, and hunger urges the cells to renewed 

 activity. Though the flagellated cells live each, as 

 has been stated, their own life, yet it is no less true 

 that each lives for the rest of the organism, as indeed 

 must happen in all organised communities. The 

 nutrition received is, under favourable circumstances, 

 more than enough to make good the loss of substance 

 involved in work, and the surplus leads to that 

 increase in size which is termed growth. 



But to increase in size there is in every individual 



