8 ARTHUE DENDY. 



shortened, while an additional fourth ray has appeared as a 

 direct continuation of the third ray in the angle between the 

 two paired rays. The cessation of growth of the third ray in 

 its usual direction is perhaps to be accounted for by the out- 

 growth of the new fourth ray in the exactly opposite direction. 

 The additional fourth ray is usually slightly hastate in form, 

 the surface of the spear-head being at the same time slightly 

 roughened. Occasionally, as shown in fig. 21, this peculiarity 

 in form is very much more strongly marked. The size of the 

 quadriradiates may be estimated from fig. 20. 



Uniaxial Spicules. 



The shape of these spicules is a modification of the oxeote 

 type, in which one end is markedly broader than the other, 

 and often decidedly hastate (figs. 10, 11). Fig. 11 represents 

 a spicule of the more usual size and form; figs. 1 , 8, 9 repre- 

 sent giant modifications of the same from the margin of the 

 osculum, and fig. 12 represents the large-sized form found in 

 the stem. All the figures are drawn to the same scale as the 

 triradiates. 



The Arrangement of the Skeleton. — In dealing with 

 this part of my subject I propose to follow the plan laid down 

 by Haeckel in his ' Monograph of the Calcareous Sponges,' 

 which seems to be in all respects the most satisfactory. 



In the Sycons Haeckel distinguishes the following skeletal 

 systems, which, for the sake of convenience, I arrange in an 

 order slightly different from his : 



1. The dermal skeleton, protecting the outer or dermal 

 surface of the tube or cup, of which the Sycon individual 

 consists. 



2. The gastral skeleton, protecting the inner or gastral 

 surface. 



3. The skeleton of the peristome, protecting the margin 

 of the osculum. 



4. The tubar skeleton, lying between the dermal and 

 gastral systems, and affording support to the flagellated 

 chambers. 



