OP THE SPONGIAD^. 135 



the ovarium ; and 3nd, those having the walls of the ovaria \ 

 supported by elongate forms of spicula, disposed on or 

 near its surface at right angles to lines radiating from the ' 

 centre to the circumference of the ovarium ; and fortu- 

 nately the types of these two forms of spicular arrange- 

 ment on the cortex of the ovarium are admirably illus- 

 trated in the two European species of Spongilla ; the first 

 mode existing in Spongilla fluviatilis, and the second 

 one in 8. lacustris. After having described the ovaria 

 of these two species as types of their respective groups, 

 I shall in my future descriptions of these organs confine 

 my observations rather to their anatomical structure 

 than to their external characters, excepting when the latter 

 are of an unusual description. These bodies occur in great 

 profusion in the basal portions of S. fluviatilis ; they are 

 spherical and of an average diameter of ^th of an inch, and 

 they are furnished with a circular foramen at their distal 

 extremity of about ^y^d of an inch in diameter. In their 

 natural condition they exhibit very slight indications of the 

 birotulate spiciila imbedded in their coriaceous-looking 

 envelope. In the dried state they become cup-shaped by 

 the contraction of the upper half inward during the process 

 of desiccation, and in this condition the foramen appears at 

 the bottom of the cup. The edges of the cup being thick 

 and round in consequence of the presence of the birotulate 

 spicula beneath the fold of the membrane, and the surface 

 becomes pitted with numerous minute lacunae, which are 

 produced by the adhesion of the inner surface of the 

 envelope to the distal extremities of the birotulate spicula. 

 Immersion in water for an hour restores them to their 

 spherical form, but does not obliterate the lacunae produced 

 by desiccation; and I have several times observed that, 

 under these circumstances, the expansion of the ova within 

 has forced one or more of them through the foramen. 



If we take several of the ovaria, either in the living con- 

 dition or in the expanded state I have described above, and 

 place them in a test-tube with a little nitric acid, and raise 

 the temperature of the whole until the ovaria becomes of a 

 bright yellow colour and semi-transparent, and then arrest 



