XIV INTEODUCTION. 



correctly determined, and the division of the genus to 

 which the specimen under examination being ascer- 

 tained we, at once, circumscribe the field of an exami- 

 nation within, comparatively, a very narrow compass. 

 The characters now called into action for specific 

 determination are principally the spicula, a series of 

 organs as variable in sizes, forms, and modes of 

 combination and of disposition in the sponge, as the 

 leaves, stipulge, &c., are in botany, and in truth they 

 are quite as important to specific discrimination in the 

 sponge, as the leaves are in the plant. In some sponges 

 we find a single form of spiculum only to prevail, but 

 in two such closely allied species having the same 

 form they frequently differ so greatly in their size, 

 proportions, and mode of disposition, as to afford 

 perfectly distinct and reliable specific characters. 

 And here the student must be warned that Canada 

 balsam and close examination with highpowers mustnot 

 be neglected, as many species, and especially so among 

 the Geodias and Tetheas, are only to be distinctly deter- 

 mined by the peculiarities of form of some of the 

 minutest of their spicula, the retentive ones. All these 

 minute forms of spicula are as constant in their struct- 

 ural forms, and as rehable as specific characters as the 

 larger forms of spicula are, and in many cases of nearly 

 allied species in which the skeleton and defensive 

 spicula very closely resemble each other, the retentive 

 ones are so strikingly different as to render the dis- 

 crimination of the species readily and certainly. It is 

 frequently the case that these minute organs in a 

 specimen immersed in water, embedded in the sarcode 

 coating the dermal or interstitial membranes, are per- 

 fectly invisible with any amount of microscopic power, 

 and it is only when mounted in Canada balsam that 

 they become visible in situ, but even then their minute 

 structural iieculiarities are not always to be distinctly 

 determined ; and it therefore becomes a necessary 

 course of proceeding to ensure the success of our 

 examination that a small portion of the sponge, includ- 



