HARD WICKE 'S S CIE NCR - G SSIF. 



■17 



THE STRUCTURE AND LIFE-HISTORY OF A SPONGE- 



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



{ContinncdfroJit page 195.] 



HE description of 

 the young sponge 

 is so far complete ; 

 it consists of a sac, 

 with a mouth at 

 one end and pores 

 at the sides, spi- 

 cules to sujjport 

 it, and with three 

 layers of tissue 

 composing the 

 wall, the ectoder- 

 mal covering of 

 plate-like cells, the 

 j elly-1 ike m e s o- 

 dermin the middle 

 and the flagellated 

 endodermal cells 

 within. But, as 

 yet, there is no 

 trace of the radial 

 tubes. Some sponges (jDlynthiis) which have the 

 same developmental history as Sycandra, up to this 

 point, remain persistently in the stage now reached 

 by it ; in the young Sycandra, however, budding 

 now begins to take place from the stomach wall, 

 little hollow processes jut out from itj as if pushed 

 out by a finger ; these grow outwards, till they 

 acquire exactly the same characters as the sac 

 •from which they proceed ; the open gastrula ends 

 correspond to the mouth of the stomach, and the 

 outer ends of the tubes to the base of the stomach. 

 These are the radial tubes, and at first they are 

 ■separate from each other, not united ; this stage in 

 the history of the sponge remains permanently 

 throughout life in a related species [S. coronata) ; in 

 our sponge, however, they soon become united by 

 transverse bars of tissue, which cross from one tube 

 to another. The ends of the tubes, however, always 

 remain free as little conical protuberances, but in 

 another sponge {S. capillosa) development proceeds 

 No. 202,— October i88i. 



the one step further, and the tubes become joined 

 right up to their extremities. The fact here illus- 

 trated, that a stage which is transitory in the history 

 of one animal is persistent in another, is one of the 

 strongest arguments " for Darwin." 



After so much pure description one may fairly be 

 allowed to indulge in a little speculation : at one 

 time people who thought at all about the matter were 

 accustomed to believe that the young animal was 

 produced from the adult all at once, at a single stroke; 

 it commenced 'as a minute germ, a more or less exact 

 likeness of the parent in miniature, which had 

 nothing to do except to grow big. Such, however, is 

 as we can see, the very opposite of being the case, a 

 vast number of phases of development intervene 

 between the fertilised ovum and the young sponge. 

 What, then, is the meaning of these phases, why all 

 this complicated process, instead of the simple 

 impress of the parental image on a young germ ? 

 The explanation which has the merit of being at once 

 the simplest and the most rational, is that the various 

 stages in the development of the individual mark 

 the various stages in the history of the species ; they 

 present us, in the course of a few days, with a sum- 

 mary very much abridged of the successive steps by 

 which the organism, as it at present exists, was 

 evolved in the course of ages from some simpler form 

 of life. Thus to confine ourselves to the history of 

 the sponge which we have now made our own, we 

 may assume that its earliest ancestor was a simple 

 cell, closely resembling an ordinary amoeba; this 

 amoeba, after leading a wandering life, feeding and 

 growing big, became stationary, folded its arms, to 

 speak symbolically, withdrew them into itself, and 

 formed a spherical ovum ; this either with or with- 

 out fusing with another individual previously, split 

 into two, as amoebas in such circumstances do at the 

 present day, but the resulting twins, instead of separ- 

 ating from one another, as ordinary young amoebas 

 do, remained in contact, for no obvious reason that 

 one can see unless to keep each other warm ; they 



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