OF THE SPONGIAD^. 315 



we must not imagine that it is very difficult to obtain these 

 characteristic evidences ; for, as I shall show more at length 

 hereafter, it requires but the dissolution of a small piece of 

 the sponge in hot nitric acid to at once furnish us with a 

 general view of the whole of the spicular contents of the 

 sponge under examination ; so that, to one who has become 

 familiarised with the general characteristics of the forms and 

 sizes of the different classes of spicula peculiar to each organ 

 of the sponge, such a preliminary observation at once indi- 

 cates the nature and especial seat of the principal specific 

 characters of the subject under examination. 



In some sponges the relative variation in size of the adult 

 skeleton spicula is greater than in others ; but this vari- 

 ation, although sometimes a substantial character, must not 

 be always assumed to be correct, as in young sponges with 

 simple forms of skeleton it is very difficult to discriminate 

 between the young and only partially developed spicula 

 and the adult ones. Thus in a young specimen of Spon- 

 gilla fluviatilis, I found in the same field of view one 

 spiculum perfectly well proportioned, which measured 

 gijth of an inch in length and To,sooth of an inch in diameter ; 

 another ^Irth of an inch in length and ^th of an inch in 

 diameter; the length and diameter of an average-sized 

 spiculum of the species in a fully developed condition being, 

 length ^th of an inch, and diameter ^^th of an inch. 



Abnormal or immature forms must not be mistaken for 

 fuUy developed and normal ones, as we find in some of the 

 more complicated forms of spicula that the development of 

 form is quite as progressive as that of size ; as instanced in 

 Figs. 73, 74, 75, and 76, Plate III, which represent the 

 progressive stages of development of the spinulo-recurvo- 

 quaternate form of spiculum, and also in Pigs. 144, 145, 

 146, and 147, Plate VI, representing the progressive de- 

 velopment of the dentato-palmate ineqiu-anchorate spiculum. 



2. The Oscula. 



The oscula frequently afford good specific characters. 

 Their peculiarities are, first, those of position ; and secondly, 



