764 
DE. J- S. BOWEEBANK OX THE AXATO]MY 
earthy deposits in the bony structures of the more perfectly developed living forms. In 
the higher tribes of animals we find the disintegrated condition of the earthy deposits in 
the first stages of the development of the bony structures in the foim of minute radiating 
patches, which in a more advanced stage unite and form the solid mass of bone, as in 
the mammalian tribes of animals, while in the cartilaginous tribe of fishes these radiating 
centres of bony secretion never attain a higher degree of development, but remain isolated 
points of bony structure during the whole of the life of the animal. And in the com- 
pound tunicated animals we find the calcareous stellate and sphero-granulate forms of 
spicula developed in close accordance with the similar siliceous forms in various species 
of sponges. Thus the stellate and cylindro-stellate spicula of the sarcode in the Spon- 
giadee are apparently the homologues of the bony centres of development in the higher 
animals. It is so likewise with the other forms of sponge spicula. AYe find isolated 
calcareous spicula of an irregular fusiformi-acerate shape, representing the bony skeleton 
of the higher animals in the outer integuments of several species of Doris. 
Messrs. Alder and Hancock, in their admirable ‘ History of the British Nudibranchiate 
Mollusca,’ describe calcareous spicula occurring in Doris aspera, hilamellata, and Triopa 
clamger, which appear to be analogous to the rectangulated-triradiate spicula of Grantia ; 
and they also state that in the first-named species crucial or dagger-shaped spicula occur 
in the branchise and margins of the cloak of the animal, and forms very similar to those 
occur on the interstitial membranes of Grantia nivea, Johnston. Numerous forms of 
tuberculated and smooth calcareous spicula are also found in the extensive family of the 
Gorgoniadac. And the siliceous simple bihamate form of retentive spiculum, so abundant 
on the interstitial membranes of many species of sponges, are closely represented by the 
calcareous bihamate spicula so numerous on the tubular suckers of Echinus sphcera. 
Thus we find in the spicula only, a series of links in the chain of animal development, 
intimately connecting the Spongiadse with the higher tribes of animals. 
In the solid siliceous fibres of Dactylocalyx., and in the tubular siliceous fibres of 
Earrea, Bowerbank, MS., and especially in the latter, we obtain a very much closer 
approximation to the tubular forms of the bones of the higher classes of animals. 
From our knowledge of the great scheme of the natural development of animal life, 
the most perfectly organized sponges appear to be those which secrete carbonate of lime 
as the earthy basis of their skeletons, and the least perfect those which secrete no earthy 
matter in the skeleton ; those which secrete silex taking an intermediate position ; but 
it must also be remembered that there is no form of spiculum found among the calca- 
reous sponges, or in the higher tribes of animal life, that is not repeated among the 
siliceous forms of spicula of the Spongiadas. 
The essential Skeleton Spicula. 
The general configuration of the spicula, which are essentially necessary to the struc- 
ture of the reticulated skeletons, is that of simple elongate and slightly curved bodies, 
varying in length and stoutness in accordance with the necessities of the structure in 
