OF THE SPONGIAD.E. 213 



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 sar- 

 code 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 deter- 

 mination of the generic and specific characters of these 

 interesting and curiously constructed animals. A careful 

 and patient examination of their component parts is there- 

 fore 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 Spongiadas 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 

 folio WS: _1. The Spicula. 2. The Oscula. 3. The Pores. 

 4. The Dermal Membrane. 5. The Skeleton. 6. The 

 Interstitial Membranes. 7-. The Intermarginal Cavities. 

 8. The Interstitial Canals and Cavities. 10. Sarcode. 

 11. Ovaria and the Gemmules. 



1. The Sjricula. 



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 preceding 

 portion of this work, 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 Pigs. 1, 2, 

 Plate I, their relative proportions are frequently so distinctly 



