OF THE SPONGIAD^E. 11 



may therefore be concluded that they contained so great a 

 proportion of calcareous matter as to prevent their disin- 

 tegration by heat. 



Many of the forms of the spicula are by no means 

 peculiar to the Spongiadse ; but, on the contrary, as I shall 

 hereafter show, their types are frequently to be found in the 

 more highly organized classes of animals, and especially 

 among the Zoophyta, the Tunicata, and the Nudibranchiate 

 Mollusca. They are always of an organic type, never 

 crystalline or angular. 



Each of the elongated forms of spicula may be said to 

 be composed of three parts, the base, the apex, and the 

 shaft intervening between the two ; and, generally speaking 

 these parts may be readily determined, even when the 

 spicula are isolated. 



Each species of sponge has not one form of spiculum 

 only, equally dispersed throughout its whole substance ; 

 but, on the contrary, we find that separate parts have each 

 its appropriate form ; and thus we find that three, four, or 

 even more forms often occur in the same individual ; and 

 in Tctkea cranium there are no less than seven distinct 

 shapes. But these differences in structure must not lead 

 us to believe that every strange form of spiculum that meets 

 the eye is a normal one ; remarkable variations are often 

 produced for especial purposes in the construction of the 

 skeleton or for other objects ; and in some species, Sjwngilla 

 lacustris, for example, the number of malformations that 

 are occasionally found is very remarkable. The size also of 

 the normal forms of spicula will often vary to a considerable 

 extent in the same sponge ; but if adult, they are always in 

 accordance with the type form, and if not adult, inter- 

 mediate states of growth are generally present to assist us 

 to form correct conclusions regarding them. The forms 

 thus appropriated to the different parts of the sponge are 

 not always peculiar to certain species, but, on the contrary, 

 they are frequently found to be repeated in other species 

 differing widely in their construction. 



The spicula thus appropriate to particular parts of the 

 sponge are uniform in their general characters throughout 



