Am) PHYSIOLOGY OP THE SPONGIAD^. 
767 
they are best developed, as in Geodia MfAndrewii Bxid. Barretti, Pachymatisma Johnstonia, 
and others of similar structure. 
I have never seen the progressive development from a simple elongate shaft of an ex- 
pando- or patento-ternate connecting spiculum, as I have those of the porrecto-ternate 
external defensive form, and the spinulo-recurvo-quaternate internal defensive ones, 
but from the great similarity that exists in their structure there can be little doubt that 
their mode of growth is the same ; and I am very much inclined to believe that the 
cylindro-expando-ternate form from Pachymatisma Johnstonia, fig. 43 in Plate XXIII. 
of the Phil. Trans, for 1858, is an incompletely developed form of the mature attenuator 
expando-ternate spiculum that belongs to that sponge, and which is represented by 
fig. 42 in the same Plate. 
There is a progression of development in the ternate terminations of these spicula 
that is very interesting. 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. 48) ; 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 ap- 
pearance, 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 never, 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, Pactylocalyx Prattii, Bowerbaxe;, MS., in 
which we have the primary radii sinuated and fiattened 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. 8, Plate XXIX., which represents a few of these spicula 
uniting to form the reticulations of the dermal tissues, while fig. 9 represents three 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 exists in the beautiful harrow-shaped tissue 
of the dermal structures of the sponge supporting the fine specimen of Puplectella in the 
possession of my friend Dr. A. Faree, and described by Prof. Owen in the ‘ Transactions 
of the Linnean Society,’ vol. xxii. p. 117, plate 21, and which tissue I shall describe more 
fully in treating on the subject of the dermal structures of the Spongiadte. 
There are two distinct purposes in the physiological application of the ternate spicula ; 
the simplest is that of the strengthening and connecting the dermal membrane with tlie 
mass of the animal beneath. The second and more complex one, is that of forming an 
internal reticulating framework for the support within its areas of the valvular tissues 
