AND PHYSIOLOGY OF THE SPONGIAD^. 
1123 
of the finer forms of stellate spicula, which are very characteristic in Tethea^ Geodia, 
Sjpongilla, and other genera. When we search for them by the dissolution of the 
tissues in nitric acid, they are so minute that by far the greater part of them, even with 
the most careful treatment, are washed away ; and when the tissues in which they are 
imbedded are examined in water, they are totally invisible in the sarcode in which they 
are immersed ; and it is only when small portions of such tissues are mounted in Canada 
balsam that they become distinctly visible in situ. The correct classification therefore, 
as well as the anatomy and physiology, is really a microscopical science ; and it is only 
since we have possessed instruments of high defining and penetrating powers, that we 
have been properly prepared for the investigation of the structures and the correct 
determination of the generic and specific characters of these interesting and curiously 
constructed animals. A careful and patient examination of their component parts is 
therefore absolutely necessary for the determination of species ; and the whole of the 
structures present should be noted and their peculiarities accurately described. 
In the first place we will consider what are the parts of the organization of the 
Spongiadae that may be used for the purposes of specific distinction; and secondly, 
endeavour to form an estimate of their relative values. 
The parts of the sponge to be thus employed are as follows: — 1. The Spicula. 2. The 
Oscula. 3. The Pores. 4. The Dermal Membrane. 5. The Skeleton. 6. The Inter- 
stitial Membranes. 7. The Intermarginal Cavities. 8. The Interstitial Canals and 
Cavities. 9. The Cloacal Cavities. 10. The Sarcode. II. The Ovaria and Gemmules. 
I. The Spicula. 
The spicula in the descriptions of the Spongiadae are of about the same relative value 
that the leaves of plants are in botanical descriptions. I have shown in the first part 
of this paper, published in the Philosophical Transactions f®r 1858, that they are 
exceedingly various in form in the different species ; and even when of the same shape 
in two different sponges, as represented in fig. 9 « & Plate XXIII. Phil. Trans. 1858, 
their relative proportions are frequently so distinctly different as to render them almost 
as valuable as if they varied from each other in form. Wherever therefore spicula form 
a component part of the skeleton, they become a leading character in the discrimina- 
tion of species. But it is not only those of the skeleton that are thus available, as in 
different sponges they vary in shape and size in each separate organ belonging to the 
animal ; and in some cases we find as many as five or six distinct descriptions of spicula, 
each of which affords an invariable and excellent character. Thus, in the descriptions 
of sponges, it is not only the forms and relative proportions of the skeleton-spicula 
which have to be taken into consideration, but those also of the dermal and interstitial 
membranes (the external and internal defensive ones), those of the sarcode, and of the 
ovaries and gemmules. Those of the latter three organs named frequently afford the 
most determinative characters. Thus in the genus Spongilla but one form of spiculum, 
the acerate, prevails in the skeletons of all the known species; but the minute and 
