Introduction to Animal Morphology. 75 



is the only one consistent with the varieties of structure 

 presented in the class. 



The plastides are usually supported on spicules of 

 calcium carbonate, silica or horny matter (Spongiolin), 

 which are formed by the deposition of the indurating 

 material in a protoplasmic base. The mineral spicules 

 are not crystalline, but laid down particle by particle, 

 often in laminae. They sometimes have a central 

 cavity filled with an organic material (protoplasm ?*), 

 for, when heated, they often burst at one end, and be- 

 come blackened inside. 



In size the spicules vary from -&" - i' (Hyalonema). 

 They are named from their shapes, and may be bi-, tri-, or hex- 

 radiate, according to the number of diverging rays ; or, if 

 quinque-radiate, may have long or short axes, and simple or 

 forked rays. Some are simply accrate (needle-shaped), pin- 

 shaped, club-shaped, fusiform, cylindrical, furcate, radiate, 

 spinulate, rotulate (wheel-like), or birotulate (like two-toothed 

 wheels joined by an axle), or sinuous. Some are hamate 

 (hooked), or C- or S-shaped ; others umbonate, anchor-like, 

 candelabriform or spheroidal. These names are sufficiently 

 ezpl The spicules are supposed to discharge different 



functions, and are grouped into: ist. Skeletal or essential, 

 supporting the body. 2. Connecting, retaining the outer 



r (when it is differentiated) in connexion with the interior. 

 3. Prehensile projecting as a means of attachment to other 

 bodies. 4. Defensive projecting externally to prevent their 

 ! upon. 5. Spicules of the surface membrane, 

 which may .strengthen or support its plastides; these may be 

 cither (a) tension spicules to keep it stretched, or (/?) retentive 

 (hamate or am liorate;, keeping it in its \> 



Reproduction tak-s place: ist. Sexually, by the- 



differentiation of some of tin- <I<-.-|HT plastid.-s, al<>n-- 



th*' lincj of the larger canals, into ova; others arc 



Candchibriftinn s]>iculcs arc solid. 



