16 POHIFERA. 



gelatinous portion continually accumulates, and, as it spreads in 

 every direction, secretes and deposits, in the form peculiar to its 

 species, the fibrous material and earthy spicula which characterise 

 the skeleton. 



(18.) From this description of the structure of a sponge, it will 

 be apparent that all parts of the mass are similarly organized : a 

 necessary consequence will be, that each part is able to carry on, 

 independently of the rest, those functions needful for existence. 

 If therefore a sponge be mechanically divided into several pieces, 

 every portion becomes a distinct animal. 



(19.) The multiplication of sponges, however, is effected in 

 another manner, which is the ordinary mode of their reproduction, 

 and forms a very interesting portion of their history.* At certain 

 seasons of the year, if a living sponge be cut to pieces, the chan- 

 nels in its interior are found to have their walls studded with yel- 

 lowish gelatinous granules, developed in the living parenchyma 

 which lines them ; these granules are the germs or gemmules 

 from which a future race will spring ; they seem to be formed in- 

 differently in all parts of the mass, sprouting as it were from the 

 albuminous crust which coats the skeleton, without the appearance 

 of any organs appropriated to their developement. As they in- 

 crease in size, they are found to project more and more into the 

 canals which ramify through the sponge, and to be provided with 

 an apparatus of locomotion of a description which we shall fre- 

 quently have occasion to mention. The gemmule assumes an 

 ovoid form, fig. 2 B, and a large portion of its surface becomes 

 covered with innumerable vibrating hairs or cilia, as they are de- 

 nominated, which are of inconceivable minuteness, yet individually 

 capable of exercising rapid movements, which produce currents in 

 the surrounding fluid. As soon therefore as a gemmule is suffi- 

 ciently mature, it becomes detached from the nidus where it was 

 formed, and whirled along by the issuing streams which are ex- 

 pelled through the fecal orifices of the parent, it escapes into the 

 water around. Instead, however, of falling to the bottom, as so appa- 

 rently helpless a particle of jelly might be expected to do, the cease- 

 less vibration of the cilia upon its surface propels it rapidly along, 

 until, being removed to a considerable distance from its original, 

 it attaches itself to a proper object, and, losing the locomotive cilia 

 which it at first possessed, it becomes fixed and motionless, and 



* Professor Grant loc. cit. 



