OF THE SPONGIAD^. 137 



treated with hot nitric acid they display an abundance of 

 short, stout, entirely spined subarcuate acerate spicula, one 

 of which is represented in Fig. 203, Plate IX. These 

 spicula are in many instances exceedingly numerous ; they 

 are disposed without order, and overlie each other at 

 various angles, forming, in their imbedment in the envelope, 

 a strong and very efficient irregular network of spicula. A 

 portion of one of these prepared ovaria is represented in 

 Mg. 320, Plate XXII. 



In the ovaries of the different species of Spongilla, to be 

 arranged hereafter in accordance v^ith these structural 

 peculiarities, there is a considerable amount of general 

 resemblance, but accompanied with such permanent 

 variations in the structure of the spicula, and in other 

 portions of the development of these organs, as to render a 

 somewhat detailed description of them necessary. Thus 

 in the development of the birotulate spicula, the ovaries 

 oi Sponffilla plumosa, Carter, exceed any other knovi^n species. 

 The thick walls of these organs are filled with them in the 

 state represented by Pig. 208, Plate IX, and the intervals 

 between their shafts appear to be filled with' indurated 

 sarcode or keratode. In Sponffilla Meyeni, Carter, the 

 structure of the walls of the ovaria are strikingly similar to 

 those of S.fluviatilis, and the form of the spicula the same, 

 with the exception of the shafts being very much more 

 spinous, and the size of the spiculum twice that of S. 

 Jluviatilis. Pig. 219, Plate IX, represents a spiculum from 

 an ovary of 8. Meyeni. The smallest and most simple 

 development of birotulate spicula exists in Spongilla ffregaria, 

 Bowerbank, from the River Amazon, represented by Figs. 

 213, 214, 215, and 216, Plate IX. 



A gradual transition from the birotulate form to that of 

 the unirotulate one takes place in the ovaries of >S'. paulula 

 (Fig. 221) and S. reticulata (Fig. 223), until we obtain the 

 perfect and beautiful unirotulate form in the ovaries of 

 8. recurvata, represented by Figs. 224 and 225 in the 

 Plate quoted above. In all these species there is a general 

 accordance in the mode of their structure. 



The gradual transition from the birotulate to the uniro- 



