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 -h" - 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 acerate (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 

 expressive. The spicules are supposed to discharge different 

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

 supporting the body. 2. Connecting, retaining the outer 

 layer (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 

 being preyed upon. 5. Spicules of the surface membrane, 

 which may strengthen or support its plastides ; these may be 

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

 (hamate or anchorate), keeping it in its place. 



Reproduction takes place : — ist. Sexually, by the 

 differentiation of some of the deeper plastides, along 

 the line of the larger canals, into ova ; others are con- 



* Candelabriform spicules are solid. 



