1128 
DE. J. S. BOWEEBANK ON THE ANATOMY 
quently peculiarly and abundantly spinous, and these secondary organs are equally as 
constant and determinative of specific character as the primary radii. The characters 
derived from the spines are frequently very minute, and require the application of a 
high microscopic power to render them available ; but they are in many cases so 
decisively valuable, that they should never be neglected when present. In truth, the 
modes of spination of these and all other forms of spicula are of considerable value as 
specific characters, and the shape and du'ection of the spines are often indicative of the 
character and purpose of the spiculum on which they are based. 
The range of the stellate spicula is very considerable. They are found abundantly 
and constantly in Geodia, Pachymatmna, Tethea, Dactylocalyx, and Alcyoncellum, and in 
some species of Sj)ongilIa, BictyocylindruSy and other genera. 
11. The Ovaria and Gemmules. 
Where the ovaria exist they afford excellent descriptive characters. Their construc- 
tion is the same throughout the whole of the known species of Geodia and Pachyma- 
tisma. The varieties in their form, although not always easy of description, are yet 
readily distinguishable by a practised eye ; and the difference in the degree of stoutness 
of the radiating spicula of which they are constructed, and the consequent fineness or 
coarseness of the reticulations on their surface, very often afford good discriminative 
characters. 
In Sjpongilla, the varieties in their shape, and the strikingly distinct forms of their 
component spicula, render them exceedingly efficient for specific descriptions ; and 
without them it would in several instances, among the exotic species, be very difficult 
to find descriptive characters to separate one species from another. 
Excepting in Biplodemia, where the structural peculiarities of the ovarium are 
widely different from the preceding instances, we know very little more of these 
organs ; but there is good reason to believe, from certain forms of spicula detected in 
the deep-sea soundings, the sources of which are at present unknown, that other marine 
sponges possess ovaria with which we are at present unacquainted. 
The gemmules afford very efficient specific characters in some species of Tethea ; but 
in the greater number of Halichondroid genera, although frequently present in abun- 
dance, they agree so closely in structure with each other as to render them of very little 
use as specific characters. 
We thus find that we possess eleven distinct varieties of organic specific characters, 
many of which are exceedingly prolific in materials for descriptive purposes. A long 
familiarity with them has assured me of their value, and of then* constancy in each spe- 
cies. However protean the form and colour may be, the organic structures can always 
be recognized with certainty, provided the specimen under examination has been dried 
in the condition in which it has been taken from the sea. To the organic characters 
may be added the less definite and valuable ones of form and mode of growth, which. 
