BY R. J. tiLLYARt>. 



55 



a layer of dust-like pigment interposed. Now an examination 

 of the alveolar wall shows (as far as magnification by a 1/1 2th 

 oil-immersion lens is able to show) a very uniform, lightly-stain- 

 ing suVjstance closely resembling the innermost stratum of the 

 cuticle, but without any pigmentation. One would not hesitate 

 to pronounce this delicate layer to be chitin, were it not for the 

 presence of the alveolar nuclei. These, however, give the key 

 to the structure. If one follows carefully through a series of 

 transverse sections of a complete gill from the base outwards, it 



^A. 



...aln 



Texl-iig.2. 

 Diagiatninatic reprcseutatiou uf formation of alveolar mcsliwork. A, the 

 two walls of the gill separated by the ha^nioca^le (Jic). B, migration of 

 hypoderni -cells {ale) into the ha?moccele, with outgrowth and union of 

 their processes. C, the alveolar meshwork completed by differentia- 

 tion of the same cells into alveolar nucleus (aht) and alveolar wall. 

 al, a single alveolus enclosing a portion of the original hiemoca'le. 

 Rest of lettering as on p. 109. 



is clearly to be seen that, near the base, a number of hypoderm- 

 cells do not lie in the regular epithelium, but project out across 

 the gill-cavity, with their nuclei placed irregularly within it. 

 These cells unite across the interior of the gill, where, indeed, at 

 the breaking-joint, they form an almost complete transverse 

 layer. Passing further into the gill, we find these same cells 

 becoming exceedingly irregular and attenuated, with the nuclei 

 few and far between, and large spaces of the luemocoele lying 



