60 ANATOMY AND PHYSIOLOGY 



the rotulae and those of the foliato-peltate spicula, but they 

 do not appear, like the latter, to be derived from the 

 ternate forms. The radiation of the canaliculi, as repre- 

 sented by Pig. 222, Plate IX, are not derived from three 

 primary rays, but each appears to emanate from a central 

 cavity at the end of the shaft; and their number, 22, 

 at their proximal termination, is not reconcilable with any 

 regular number of bifurcations arising from three primary 

 rays, however short we may imagine them to be. 



The progressive decline of the inner rotula in the inequi- 

 birotulate spiculum of Sponyilla paulula, Bowerbank (Pig. 

 221, Plate IX), and its all but total extinction in SpongiUa 

 reticulata, and Sponcjilla recurvata, Bowerbank (Pigs. 223 

 and 224), until the distal rotula merges in the scutulate 

 form, with an acute external umbo in place of an internal 

 shaft as in Spongilla Brownii, Bowerbank, Figs. 226 and 

 227, exhibits a very interesting series of gradations of 

 development in the same description of organ. 



The whole of this beautiful group of spicula occur in 

 the thick coriaceous proper coat of the ovaria of the Spon- 

 gillidae. Sometimes we have but one form thus located, as in 

 Spongittajluviatilis, Johnston, where we find them very close 

 together in the case of the ovariurn, as in Pig. 318, Plate 

 XXII, the outer rotula supporting the external membrane, 

 and the inner one performing the same office for the internal 

 one, as represented by Pig. 319, Plate XXII. At other times 

 we find two distinct forms in the coat of the ovarium, as in 

 Spongilla recurvata, Bowerbank, from the River Amazon ; 

 the inner one being slender boletiform (Pig. 224, Plate IX), 

 and the outer one multihamate birotulate (Fig. 220, 

 Plate IX). In every case these spicula are so completely 

 immersed in the thick coriaceous coat of the ovarium, that 

 they are perfectly invisible under ordinary circumstances ; 

 and it is only after the ovary has been boiled in nitric acid 

 for a very short period, that it is rendered sufficiently 

 transparent to allow of the spicula being seen in situ. 



The progressive development of these forms of spicula is 

 very beautifully exhibited in the spicula from the ovaria of 

 Spongitta plumosa, Carter. We first observe them, with a 



