58 ANATOMY AND PHYSIOLOGY 



the comparatively thick crust of the ovarium, their long 

 axes being always at right angles to lines radiating from 

 its centre to its circumference. Their forms become 

 widely different from those of the skeleton spicula, and 

 especially adapted to their peculiar office ; and their 

 terminations frequently expand into broad plates, as in 

 Sponyilla Jluviatilis, Johnston. Their forms vary con- 

 siderably in shape and structure in different species. In 

 the ovaries of some sponges, one of these modes of the 

 disposition of their spicula only can be observed. 



In the third mode of arrangement, where the spicula 

 abound in every part of the gemmule, as in Tethea cranium, 

 Johnston, they are various in form, but resemble to a 

 considerable extent those of the skeleton, with an inter- 

 mixture of forms peculiar to the gemmule. 



In ftpongilla Carteri, Bowerbank, and S. Jluviatilis, 

 Johnston, our commonest British species, belonging to the 

 first group, the external series of spicula of the ovaria are of 

 the same form as those of the skeleton, but frequently 

 somewhat shorter. They are disposed irregularly over the 

 surface of the ovarium, and firmly cemented to it by 

 the middle of the shaft, while each of their apices are 

 projected in tangental lines. Thus their shafts perform 

 the office of tension spicula, while their terminations 

 become efficient weapons of defence. Fig. 201, Plate IX, 

 represents the spiculum of the ovary of $. Carteri. 



In other cases in this group we find these spicula 

 differing from those of the skeleton of the parent sponge ; 

 thus the one represented by Fig. 203, Plate IX, from the 

 surface of Spo ng ilia lacustris, Johnston, is curved so as 

 to accommodate it to the rotundity of the ovary (Fig. 320, 

 Plate XXII), and we do not find its apices projecting 

 as in those of S. Jluviatilis, but instead of the projecting 

 apices, the whole spiculum is covered with minute spines, 

 assimilating it in character with the general structure 

 of those spicula which combine the office of tension and 

 defensive spicula, but differing considerably in their pro- 

 portion from the tension spicula of the same sponge, 

 S. lacustris, represented by Fig. 90, Plate IV, the one 



