814 
DE. J. S. BOWEEBANK ON THE ANATOMY 
dispersed amid the interstitial tissues, but principally towards the base of the sponge, 
while in Geodia and ddachymatisma they are congregated in large quantities immediately 
beneath the dermal membrane ; and when they have shed their ova they permanently 
retain their situation, forming a thick crustular dermis for the protection of the softer 
portions beneath : a few only are found dispersed in the interstitial membranes of the 
sponge. The progressive development of this kind of ovarium is very nearly the same 
in every species of Geodia or Pachymatisma in which I have had an opportunity of 
examining them. In an early stage they appear as a globular body of fusiformi-acerate 
spicula, radiating regularly from a central point in the mass. As the indmdual spicula 
increase in diameter there is a corresponding distention 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 deve- 
lopment 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 fiat 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 opake 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 terminations of the spicula which have not been cemented 
together like their distal ones, are progressively and simultaneously 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 con- 
tinued until the whole of them have been ejected, and the ca^ity becomes completely 
filled by the continued encroachment of the proximal ends of the spicula of the walls 
of the ovarium. 
In fig. 6, Plate XXXIV., two ovaries from Geodia IPAndrewii containing ova are repre- 
sented: {a) contains about the greatest quantity of ova that is found within these organs. 
In this one the distal terminations of the spicula of the skeleton are still somewhat 
rounded, and slightly elevated above the common surface ; Avhile in (i), Avhich has been 
partially exhausted of the ova, the spicula have their distal terminations flat and some- 
what angular, and they are level with the general surface, thus indicating a greater age 
and a fuller development than obtain in the one represented by (a), and not a less 
amount of secretion of ova, as might possibly be imagined. These circumstances are 
