HARD WICKE'S SCIENCE- G OSSIF. 



193 



THE STRUCTURE AND LIFE-HISTORY OF A SPONGE. 



By professor W. J. SOLLAS, M.A., F.R.S.E., F.G.S., &c, 



[Continued from page 159.] 





; will now resume 

 our description of 

 the structure of 

 the sponge. 



The Skeletojt. — 

 The soft tissues of 

 the sponge require 

 some kind of sup- 

 port, and this is 

 afforded by the 

 hard parts, or 

 skeleton. This 

 consists of needle- 

 shaped, t r i - 

 radiate, and 

 quad r i -radiate 

 spicules, disposed 

 in a definite man- 

 ner. The proper 

 wall of the sto- 

 mach is furnished 

 with three and four-rayed spicules, and three of the 

 rays of the quadri-radiate, and all those of the tri- 

 radiate spicules, lying in the substance of the wall, 

 parallel to its surface, while the fourth ray of the 

 quadri-radiate spicules projects with a gentle upward 

 curve into the gastric cavity, carrying the gastral 

 membrane with it. 



The radial tubes are furnished with tri-radiate 

 spicules, arranged in successive, concentric, or trans- 

 verse rows. The form of each spicule is such that 

 two of its rays, those including the largest angle, 

 may be taken to form a pair, the third ray being 

 therefore * ' odd ; " and they are so arranged that the 

 paired rays lie concentrically, while the odd ray lies 

 longitudinally, in the wall of the tube. The paired 

 rays, which form the basal or proximal row of each 

 radial tube, lie back to back, as it were, with the 

 spicules of the stomach wall, which is thus doubly 

 strengthened. 



The outer ends of the radial tubes are furnished 

 in addition with colossal fusiform spicules, each often 

 No. 201.— September i88i. 



mm. long ; 



these are imbedded at one end in the 

 tissue of the tube, and at the other project freely 

 beyond it ; about the base of each colossal spicule 

 there is usually a pencil of similar, but smaller 

 spicules, and a few large grapnel-like spicules are 

 also present. The spicules, large and small, thus 

 projecting from the end of the tube in the form of a 

 brush or pencil, give to the surface of the sponge the 

 hairy appearance to which attention has already been 

 directed. 



The spicules of the corona are partly colossal 

 spicules similar to the foregoing, partly tri-radiate 

 and quadri-radiate spicules, with three rays of the 

 latter and two of the former imbedded around the 

 mouth, the remaining ray in each case projecting 

 vertically upwards. 



This nearly completes our account of the structure 

 of the sponge ; that which remains, the nature of the 

 reproductive organs, will form a natural introduction 

 to the account of its life-history. 



Certain amoeboid cells have already been described 

 as inhabitants of the mesoderm, and the origin, or 

 probable origin, of these cells has also been pointed 

 out. What the subsequent history of them all may 

 be we do not yet surely know, but some of them, at 

 all events, continue for some time wandering through 

 the mesoderm, and instead of contributing food to 

 the rest of the sponge, obtain their nutriment from 

 it, like parasites ; thus they grow big, at the same 

 time they become lazy, and at length cease to move 

 about at all ; then they assume a spherical form, and 

 remain stationary in a cavity of the mesoderm. If 

 they now retain their simple cell-form, simply in- 

 creasing in size, they are known as ova. Some of 

 them, however, undergo a structural change (at least, 

 so we infer, for this is one of the stages in the history 

 of the sponge which has not yet been directly 

 observed), and as a result of this change, the nucleus 

 of the cell disappears, and the cell itself becomes 

 fibrillated, the fibres all radiating from the centre of 

 the cell to the exterior. It is now called a sperm-ball. 

 When it becomes mature, the fibres within are set 



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