28 DiCTYOSPONGIDiB. 



DicrrYosPONOiD.fi, but also to a correct orientation of certain of its members. 

 The "'enera PuKAfnionirTYA and Tiiy'sanodictya, which are here fully described 

 and illustrated, are subcylindrical sponges having the margin at one extremity 

 ret'ular, unmodified and free ; the other extremity is an almost transverse 

 plate, septum or diaphragm which completely closes the cup, and about its 

 edges where in contact with the walls of the cylinder, is a free expansion or 

 frill. In EuPLECTELLA, which is throughout the nearest living representative 

 of the Dictyosponges, there is, at the v/pper extremity a tei-mlnal plate, the 

 so-called *' sieve-plate," surrounded by a free marginal frill of spicular net- work.* 

 This terminal portion of the skeleton is, however, less a perforated plate than 

 a coml)ination of stout, inosculating rods or trabecidae. It is described by 

 SciiULZE in the following terms : " The watch-glass-like, arched, terminal sieve- 

 plate consists of a lattice-work of laterally compressed ridges oi various 

 thickness, which, though exhibiting no very regular arrangement, yet suggests 

 a wheel-like reticular structure. One can distinguish at least three or four 

 main beams which are approximately cii-cular and several which extend 

 radially. These form the primary meshes, ^vhich are again divided by narrower 

 and less prominent ridges. Here and there a broader plate is formed in the 

 net-work, as if by the confluence of the stronger beams."f 



This structure is in no respect similar in composition to the net-work 

 constituting the rest of the sponge and its beams are not continuous exten- 

 sions of the ground-work strands of the latter. It appears to be ^vholly a 

 protective apparatus for the oscuhun, cemented to the upper mai'gin of the 

 reticulum. The marginal frill in Euplectella is simply one of the many 

 surface ridges covering the outer wall of the sponge. 



Ill Piiragmodici'ya and Thysanodictya the structure of the terminal 

 plate is wholly different. It is, pi-imarily, a low cone, and is constituted of 

 radial spicular strands which are an immediate continuation of the vertical 

 fascicles of the reticulum, those being crossed by concentric strands, corre- 

 sponding to the horizontal bundles of the rest of the skeleton. The radial 

 strands converge at or near the center of the cone, and when the outer part of 

 the cone is preserved there is evidence of the existence of a spicular tuft 

 about the apex. Not all the rods of the vertical bundles extend from the l)ody 

 of the sponge into this plate ; a portion of them is continued into the broad 

 thin peripheral frill, the surface of which is crenidated, while its distal margin 

 is irregularly fringed by long radiciform extensions. The radial spicules of 



•OwKN at first regarded this sieve-ptate as at the bottom of Eui'LKCTKLla (Sec Sciiui.zk, op.cit. y. 54). 

 t Challenger Expedition, vo). xxi ; Hexactiaellida, p. 66. 



