OF THE SPONGIAD^. 189 



the ovary are much greater in length than in thickness. 

 The spicula of the case are disposed in a close and irregular 

 network, seldom exceeding two spicula in thickness. By a 

 careful treatment with hot nitric acid, the thick coriaceous 

 outer portion of the ovarium may be removed, and its thin 

 lining membrane, with its stratum of umbonato-scutulate 

 spicula, becomes an exceedingly beautiful object. The same 

 mode of operation displays the structure of the reticulated 

 case of the ovary very much more distinctly than when 

 viewed in its natural condition. Eg. 321, Plate XXII, 

 represents two of the cases after treatment with acid, one of 

 them [b) having the ovary very much reduced in size by the 

 dissolution of the thick coriaceous portion of its structure. 



In the second group of the ovaries of the Spongillidse 

 there is also a strong general resemblance in structure to 

 the type-form of 8. lacustris, but each species is distinctly 

 characterised by peculiarities of form and arrangement of 

 the spicula. 



The normal form is spherical, and the walls of the ovaries, 

 in six out of the seven species with which I am acquainted, 

 are comparatively thin. In the seventh species, 8. Carteri, 

 Bowerbank [8. friaUlis, Carter), they are very thick and 

 abundantly furnished with cellular structure, arranged in 

 lines radiating from the centre to the circumference ; each 

 line consists of nine or ten cells, the length of each being 

 about equal to the diameter. They are very closely packed 

 together, and are irregularly angular by compression. Their 

 combined length varies from about one-fifth to one-sixth 

 the length of the diameter of the ovarium. This is the 

 only species in which I have detected this description of 

 cellular structure. Fig. 284, Plate XVI, represents a portion 

 of the surface and a view of the cells in situ. 



Although the spiculated coriaceous form of ovarium pre- 

 vails so constantly among the freshwater sponges, it is one 

 of extremely rare occurrence among the marine species ; 

 and I have met with only one instance of its occurrence, and 

 that is in a new genus of sponges from Shetland, for which 

 I am indebted to my late indefatigable friend Mr. Barlee. 

 The specimen incrusts a portion of the valve of a Pecten, 



