1920] 



Skeletal Element in Lithistid Sponges 



57 



substance, "axial canal", retains its former size (cf. Figs. 1, 2), thus 

 extending only into the basal part of each ray. This is the character- 

 istic behavior of the axial canal in the growth of the lithistid desma 

 in general. 



As such desmas develop their final shape, they become firmly united 

 to the skeletal framework already formed. If one wishes to stud}' 

 accurateh' their fundamental shape after union, the spicules must be 

 isolated through the application of hydrofluoric acid to rough slices 

 of the framework. When the framework is so treated the desmas 

 fall apart. They are however corroded. 



Figs. 1-4. Jereopsis fruticosa. Fig. 1, a free and uneorroded amphitriaene 

 from the ectosome. Fig. 2, a young desma, slightly corroded by hydrofluoric 

 acid, from the ectosome. Figs. 3 and 4, adult desmas somewhat corroded by the 

 hydrofluoric acid used to dissociate them from the skeletal framework. All x 150. 



Desmas (Figs. 3, 4) obtained in this way show the unchanged 

 axial canal system which indicates the shape of the basic spicule on 

 which the desma has been built up. The shaft is now about three 

 times as thick as in the original spicule but no longer. The rays, 

 better designated now as branches (technically cladi), vary a great 

 deal not only in different spicules but in the same spicule. In some 

 cases they have not advanced over the condition described for the inter- 

 mediate stage (Fig. 2), either in size or complexity. More often, the 

 branch is itself branched, a condition which is produced, of course, not 



