576 



crustular coat of the sponge ; while the stout and elongated shaft is 

 intermingled with, .and firmly cemented by, keratode to the general 

 mass of the skeleton. 



3rd. The defensive spicula are divisible into two classes : those 

 of the exterior, and those of the interior of the sponge. They are 

 neither of them necessarily present in every species, nor are they 

 confined to particular genera, but occur occasionally, and in certain 

 species of various genera apparently as the necessities of the animal 

 may render their presence requisite. Their office is evidently to 

 defend the sponge from the attacks of predacious animals. They 

 are projected for about half or two-thirds of their length at various 

 angles from the surface of the sponge, or they are based on the 

 fibre of the skeleton, and are projected at about right angles into its 

 interstitial cavities. 



4th. The spicula of the membranes are of two distinct classes. 

 The office of the first of these is to strengthen and support those 

 delicate tissues, and to communicate to them a certain amount of 

 tension. The forms are few in number, and their structure com- 

 paratively simple. The office of the second class is that of assisting 

 in the retention of the sarcode on the interstitial and other struc- 

 tures. They are usually minute in size, and often very complicated 

 in form. 



5th. Spicula of the sarcode. The numerous and beautiful tribe of 

 stellate spicula appear to be devoted to connect and give substance to 

 the gelatinoid sarcode which so abundantly covers the whole of the 

 interior membranous structures of the sponges in which they occur. 

 They are often exceedingly minute, and are occasionally remarkably 

 complex and beautiful in structure, and we frequently find more 

 than one form imbedded in the sarcode of the same sponge. 



6th. The spicula appropriated to the gemmules of sponges occur in 

 various modes of disposition. First, they are imbedded irregularly in 

 an external envelope of the gemmule, or on the surface of the 

 gemmule itself at right angles to lines radiating from its centre. 

 Secondly, they are arranged symmetrically in the crust of the 

 gemmule parallel to lines radiating from its centre. Thirdly, they 

 are disposed in fasciculi in the substance of the gemmule from the 

 centre to the circumference. 



The forms occurring in the second class of these spicula are 



