AjJ^D physiologt of the spongiad^. 
785 
The progressive decline of the inner rotula in the ineqni-birotulate spiculum of Spon- 
gilla jpaulula (fig. 31), and its all but total extinction in Spongilla reticulata and Sponr 
gilla recurvata (figs. 33 & 34) until the distal rotula merges in the scutulate form, 
with an acute external umbo in place of an internal shaft as in Spongilla Brownii, figs. 
36 & 37, Phil. Trans. 1858, exhibit a very interesting series of gradations of develop- 
ment in the same description of organ. 
The spicula of the third group (those having the spicula disposed in fasciculi in the 
substance of the gemmule) differ less in character from those of the parent sponge than 
either of the preceding groups. They are in reahty but modifications of the external 
defensive spicula of the parent sponges. 
The inequi-fusiformi-acerate one (fig. 39, Plate XXVI. Phil. Trans. 1858) differs from 
the fusiformi-acerate one of the skeleton in no other respect than in the greater propor- 
tionate attenuation towards its distal termination, which gives it a degree of flexibility 
that allows of its bending freely under the pressure of any comparatively large body ; 
and I have seen them, when two gemmules have been pressed closely together, bent to 
the extent of semicircles without breaking. In the young gemmules these spicula are 
usually projected much beyond the other forms of defensive spicula that accompany 
them. 
In like manner the small attenuato-porrecto-temate form (fig. 43, Plate XXVI. Phil. 
Trans. 1858) is a modification of the similarly formed external defensive spicula of the 
parent sponge. In the adult gemmule the apices of these spicula rarely project beyond 
the dermal membrane, and it is only on pressure from without that they would be 
brought into effective use. The amount of the angle of their radiation at the apex of 
the spiculum is therefore greatly increased beyond those of the external defensive ones 
of like form in the parent sponge, so as to accommodate their apices to the curve of the 
smTace of the gemmule, and to render each point equally effective ; and as they are not 
projected beyond the dermal surface, as in the sponge, their shafts are shortened pro- 
portionally. 
The unihamate, bihamate, and recurvo-ternate forms of the same gemmules (figs. 40, 
41 & 42, Plate XXVI. Phil. Trans. 1858) are also modified forms of the recurvo- 
ternate external defensive spicula of the parent sponges, Tethea cranium and simillimus. 
Of the other torms of “ spicula the position of which are unknown,” I can say little 
more than I have before stated, excepting that I have since found the subspinulato- 
arcuate one, represented by fig. 51, Plate XXVI. Phil. Trans. 1858, in situ in a new 
species of sponge firom Freemantle, Australia, Hymeniacidon Cliftoni, Bowekbank, MS., 
and that it is a retentive spiculum. 
The Interstitial Canals and Cavities. 
These organs exhibit their most complete mode of development in the genus Sjgongia 
and in the Halichondroid sponges, occupying nearly the whole of the masses of the 
animals. They consist of two distinct systems, an incurrent and an excurrent one. 
