66 



MANUAL OK ZOOLOGY. 



have been shown by Mr Carter to be the essential element in 

 the organisation of the fresh-water and marine sponges, and to 

 be the fundamental expression of the alimentary system. 



The reproduction of sponges may be effected either 

 asexually or sexually, the following being a brief outline of 

 the phenomena which have been observed in the common 

 fresh-water sponge {Spongilla), in which the process has been 

 most accurately noticed. 



Fig. Q — a Gemtnule of Spongilla ; h Hilum ; b Diagrammatic section of the gem- 

 .mule, allowing the outer layer of amphidiscs and the inner mass of cells ; c One 

 of the amphidiscs seen in profile ; d Fragment of the skeleton of a horny sponge 

 (after 3owerbatik\ showing the interlacing homy fibres with spicula. All much 

 magnified. 



In the first or asexual method of reproduction, which takes 

 place in the winter, the deeper portions of the sponge are 

 found- to be filled with small seed-like rounded bodies, termed 

 "gemmules" or "spores," each of which possesses a small 

 aperture of " hilum " at one point (fig. 9, h). Each gemmule 

 is composed of an outer coriaceous capsule surrounded by a 

 layer of peculiar asteroid spicula, resembling two toothed 

 wheels united by an axle, and termed " amphidiscs " (fig. 9, b, 

 c). These amphidiscs are embedded ih sarcode, whilst their 

 inner surfaces rest upon the tesselated capsule already men- 

 tioned. In the interior of the capsule thus formed is a mass 

 of cells, of which the central ones contain numerous germs. 

 When the spring comes, these masses of " ovi-bearing cells " 

 are discharged through the " hilum " of the gemmule into the 

 water, and are developed into new SpongillcR. 



Spongilla also appears to reproduce itself in a somewhat ana- 

 logous manner by means of what are termed "swarm-spores," 

 These are small bodies, containing reproductive germs, and 



