OF THE SPONGIADjE. 141 



examining them. In an early stage they appear as a 

 globular body of fusiformi-acerate spicula, radiating regu- 

 larly from a central point in the mass. As the individual 

 spicula increase in diameter there is a corresponding disten- 

 sion of the ovarium, and as the spicula do not lengthen 

 in proportion to their increase of diameter a central cavity 

 is produced, in which the incipient ova very shortly appear. 

 The spicula of the wall of the ovary continue to increase 

 considerably in diameter, but very little in length, and their 

 distal terminations become gradually less acute as they 

 approach the period of the full development of the ovary. 

 When this organ has attained its greatest diameter, their 

 distal extremities cease to lengthen, and a gradual change 

 in the form of the spicula is effected, their apices extending 

 in diameter and assuming a truncated form, and the whole 

 of them becoming firmly cemented together, so as to form 

 a common flat smooth surface to the siliceous skeleton of 

 the ovarium, each spiculum having now changed from the 

 acerate to the acuate form, their proximal acute terminations 

 forming the common inner surface of the cavity of the 

 ovarium, which is now filled with an opaque mass of ova. 

 A single conical orifice or foramen has also been produced 

 in a portion of the wall, through which the ova are destined 

 to be ejected. The proximal end of this foramen is very 

 much the smaller of the two, so that, as soon as an ovum 

 has fairly entered this conical tube, there is no longer any 

 impediment to its ejection -. and the manner in which this 

 is effected is very interesting, and appears to be as follows. 

 When the ova have attained maturity, the proximal termi- 

 nations of the spicula which have not been cemented 

 together like their distal ones, are progressively and simul- 

 taneously lengthened, thereby encroaching on and gradually 

 lessening the diameter of the cavity within, so that the ova 

 are compressed and forced through the foramen ; and this 

 process appears to be continued until the whole of them 

 have been ejected, and the cavity becomes completely filled 

 by the continued encroachment of the proximal ends of the 

 spicula of the walls of the ovarium. 



In Fig. 327, Plate XXIII, two ovaries from Geodia 



