90 I. IJIMA : HEXACTINELLIDA. I. 



5 mm. ill width. Others separated by a prominent parietal ledge 

 are frequently 8 mm. or more apart. 



The parietalledffc is very well developed and gives a strikingly 

 characteristic appearance to the sponge. It is bi'oad and generally 

 round-edged but quite irregular as to the course and configura- 

 tion it takes. Sometimes it remains rather low and exhibits an 

 approach to a checkered arrangement in that it runs in inter- 

 secting transverse and longitudinal systems (see fig. 2, PI. TIT). 

 Much more usually the ledges rise in irregular crests, lappets or 

 tubercles of variable height and extent. These present an 

 appearance on the whole quite different from those of IJ. imperialis 

 in being broader and more bold in their outlines. The crests 

 may pursue a plainly oblique course after the manner of those 

 in E. aspergilluin (fig. 1, PI. Ill), but this is by no means 

 general. In fact, they may run in any direction, often tortuously 

 and as often branching and anastomosing in an altogether in- 

 definite manner. They are generally in greatest development 

 where the sponge-body is broadest. Here they may be 10 mm., 

 sometimes even 15 mm., high, as measured above the level of 

 the parietal oscula. Close to either end of the body, the ledges 

 are on the whole low, though by no means uniformly so at 

 different points or in different individuals. 



Originally separate parts of adjacent ledges may, during 

 growth, come into contact and fuse together. In this way is to 

 be explained the origin of the bridge-like connections which have 

 been now and^then observed. The growing ledge may occasion- 

 ally so encroach from all sides upon the position of a parietal 

 oscula, that there finally arises a narrow canal opening externally 



