50 ANATOMY AND PHYSIOLOGY 



distinctly visible. They occur in all parts of the sarcodous 

 surfaces of the interior of the sponge, and are frequently 

 found in greater profusion than usual on the inner or 

 sarcodous surface of the dermal membrane ; but I do 

 not recollect an instance of their occurrence on the outer 

 surface of that organ, while on the sarcodous or interstitial 

 membranes they are frequently to be observed in about 

 equal proportions on both sides of the same membrane. 



It will not be necessary to describe or figure the whole 

 of these variable forms of spicula. I have therefore selected 

 those only that may be considered more especially as type 

 forms. 



Spicula of the Sarcode. 



As the tension spicula of the membranes are destined to 

 strengthen and support those tissues, so the numerous and 

 beautiful tribe of stellate spicula appear to be devoted to 

 connect and give substance, and in some instances to 

 defend the gelatinoid sarcode, which so abundantly covers 

 the whole of the interior membranous structures of the 

 sponges in which they occur. It is difficult at first sight 

 to determine the difference in the office of this class of 

 spicula, and those of the internal retentive ones ; and it is 

 probable that in some cases, when it so happens that the 

 radii of the stellate forms rest on, and become cemented to 

 the membranous structures, they may perform, to a certain 

 extent, the same function, that of assisting to connect the 

 membranes and sarcodous structures more firmly together. 

 But generally speaking this is not the case, and especially 

 with the smaller forms of these organs ; for in compara- 

 tively thick layers of sarcode we find them in all parts, and 

 manifestly unconnected with the membranes beneath ; 

 and in sponges which have undergone such an amount 

 of decomposition as to leave the membranous structures 

 entirely, or very nearly, free from sarcode, while we see 

 the retentive forms remaining firmly attached to the 

 membranes, we rarely find the stellate ones, excepting 

 when entangled among the surrounding spicula of the 



