26o McMURRICH, [Vol. II. 



obliquely. The arrangement of the ciliated cells in bands is 

 clearly seen, the bands being much wider in comparison with 

 the bands of intermediate tissue than in the last section, indi- 

 cations, indeed, of their continuity being shown by the short 

 but narrow and closely packed cells which line the surface of 

 each intervening ridge. Finally, it may be stated, in a section 

 still higher up one finds a perfect continuity of the bands of 

 ciliated cells, the section cutting the marginal ciliated cells 

 longitudinally. 



The general structure just described is essentially that 

 described by previous authors, and more especially by von 

 Heider ('95). My interpretation of the various parts differs, 

 however, somewhat from that given by von Heider. He rec- 

 ognizes the intermediate epithelium, but regards it as an endo- 

 dermal layer separating the marginal ciliated ectoderm from the 

 median lobe of the filament, which he identifies with the glan- 

 dular streak of the hexactinian filament. I shall return to a 

 discussion of the nature of the epithelium of the median lobe 

 later ; in the mean time I may point out what seems to me a 

 fundamental error in von Heider's interpretation. He regards 

 the entire intermediate region of the wings as digestive in 

 function, terming it the " Driisenwulst," and identifying it with 

 the endodermal areas of the glandular streaks which Willem ('93) 

 has shown to be digestive. 



As a matter of fact, there are two very different kinds of 

 epithelium in this intermediate region; (i)that lining the furrows 

 which run across it, and (2) that occupying the intervals between 

 the furrows. The former is exactly like the epithelium found 

 at the free margins of the lamellae, and is, indeed, continuous 

 with this. In other words, the ciliated epithelium, which forms 

 a continuous streak near the free edge of each lamella, sends 

 inwards almost to the free edge of the mesentery a number of 

 prolongations which line the bottom of depressions on the sur- 

 face of the lamella. Each of these prolongations is separated 

 from its neighbors by a non-ciliated band (at least, I have not 

 been able to detect cilia in my preparations) of epithelium. It 

 is this arrangement of the ciliated cells in transverse streaks 

 which produces the characteristic transversely striated appear- 



