140 ANATOMY AND PHYSIOLOGY 



covering a space about half an inch in length and the eighth 

 of an inch in breadth, and it does not exceed half a line in 

 thickness. The ovaries are numerous and closely packed 

 together, and are distinctly visible to the unassisted eye, 

 looking like very minute cocoons of some terrestrial insect. 

 There were nearly thirty in an area equal to about a quarter 

 of an inch. They are attached by the sides to one or more 

 branches of the fibrous portion of the skeleton. 



The wall of the ovary is very thin, and appears to consist 

 of a single membrane profusely furnished with acerate 

 spicula, like those of the skeleton. They cross each other in 

 every possible direction, and occasionally appear to assume 

 a somewhat fasciculated arrangement. The ovaries are not 

 uniform in shape, some being regularly oval, while others 

 are more or less ovoid. I could not detect any trace of a 

 foramen in those I subjected to examination. I have 

 designated this interesting species Diplodemia vesicula in 

 my description of it. Fig. 324, Plate XXIII, represents 

 two of the ovaries in their natural condition after immersion 

 in Canada balsam, magnified 83 linear. 



In the genera Geodia and Pachymatisma ovaria are pro- 

 duced in great abundance. They agree in form very closely 

 with those of Spongilla, but their structure is widely 

 dififerent, and the soft animal matter that enters so largely 

 into the structure of those of the freshwater sponges scarcely 

 makes its appearance in the ovaries of Geodia, their walls 

 being composed of closely packed spicula, firmly cemented 

 together by silex. Their situation in the animal is also differ- 

 ent from those of Spongilla, in which they are dispersed amid 

 the interstitial tissues, but principally towards the base of the 

 sponge, while in Geodia and Pacligmatisma they are con- 

 gregated in large quantities immediately beneath the dermal 

 membrane ; and when they have shed their ova they per- 

 manently 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 



