STUDIES ON THE COMPARATIVE ANATOMY OE SPONGES. 11 



tubar skeleton is always composed of triradiates, and is dis- 

 tinguished by the fact that along the length of the chamber 

 there are always two or more transverse zones or '^ joints " of 

 triradiatesj one behind the other. The triradiates are usually 

 sagittal, and the shaft is directed towards the dermal surface. 

 The longer the flagellated chamber the greater is the number 

 of joints in its skeleton. 



Haeckel adds that in most Sycons with an articulate tubar 

 skeleton the separate joints of the latter become specially 

 differentiated. Thus the first or innermost joint is longer 

 than the following ; the outermost, on the other hand, is the 

 shortest. The triradiates of the first joint, again, are most 

 markedly sagittal, while their basal ray is unusually elongated, 

 and their paired lateral rays are placed with convex, curved 

 margin beneath the gastral surface. The triradiates of the 

 following joints are usually less markedly sagittal, their basal 

 ray less hypertrophied, and their oral angle generally smaller. 

 Finally, at the distal end of the tube, towards the dermal 

 surface, the sagittal triradiates generally pass over into the 

 regular or subregular, and often into the irregular form. 



The tubar skeleton in Grantia labyrinthica (fig. 22) is 

 articulate, and agrees very exactly with the general description 

 of such a skeleton given by Haeckel. The first joint is much 

 longer than any of the others_, and the sagittal triradiates 

 composing it ( = subgastral triradiates of Haeckel) are modified 

 precisely as he describes, their short, curved, lateral arms 

 lying beneath, or, indeed, forming a portion of, the gastral 

 skeleton. The number of joints depends upon the length of 

 the flagellated chamber, and this again varies with its position 

 in the sponge, the older and longer chambers being situate 

 nearer the base, and the younger and shorter ones nearer the 

 margin of the cup. 



It is hoped that fig. 22, which is drawn on the same plan as 

 that adopted by Haeckel, will make all these points clear 

 without further description. 



It remains to be added that the two shorter arms of each 

 triradiate curve slightly towards one another, so as partly to 



