18 ANATOMY AND PHYSIOLOGY 



mode of growth is the same ; and I am very much inclined 

 to believe that the cylindro-expando-ternate form from 

 Pachymatisma Johnstonia, Fig. 46, Plate II, is an incom- 

 pletely developed form of the mature attenuato-expando- 

 ternate spiculum that belongs to that sponge, and which is 

 represented by Fig. 45 in the same Plate. 



There is a progression of development in the ternate 

 terminations of these forms of spicula that is very interest- 

 ing. The simplest form has three nearly straight attenuating 

 radii. In the next stage the distal ends of the primary 

 radii become furcated, but the secondary radii remain in 

 the same plane as the primary ones. In the third stage of 

 development the terminations of the secondary radii again 

 divide into furcations, becoming dichotomo-patento-ternate 

 (Fig. 53), but in this case the radii of the extreme furcations 

 are not all in the same plane, as appears always to be the 

 case with those of the secondary radii, and thus we have 

 produced an additional power for combined action. But in 

 the whole of these varieties, in the structure of these ternate 

 terminations, hitherto there is no appearance, further than 

 their general form, of their being destined to become a 

 united structure, and in some sponges in which they do 

 occur they rarely, or ever do, become thus united ; but this 

 demonstration of their destination for combined action is 

 obtained in an irregular ternate form, as exhibited in the 

 dermal structures of a new species of siliceo-fibrous 

 sponge from India, Dactylocalyx Prattii, Bowerbank, MS., 

 in which we have the primary radii sinuated and flattened 

 in such a manner as to splice together and form a strong 

 and regular reticulated structure for the support of the 

 dermal membrane of the sponge, as in Fig. 300, Plate XX, 

 which represents a few of these spicula uniting to form the 

 reticulations of the dermal tissues, while Fig. 52, Plate II, 

 represents one of these spicula separated by boiling 

 nitric acid. By this structure, as exhibited in D. Prattii, 

 there is rendered apparent a more visible and common 

 purpose in their form and mode of development, and we 

 are gradually conducted to the still more complete and 

 continuous form of fibro-siliceous dermal network that 



