54 CAUDAL GILLS OF ZYGOfTERlD LARV^, 



5. The tufts figured in Ris' photograph, shown on Plate iv., 

 fig. 33, undoubtedly extend just beyond the limits of the alveolus 

 shown. 



6. Ris' own photographs show a structure quite unlike any- 

 thing known to occur in the animal kingdom, and bearing on 

 their very face the stamp of artefacts. 



I may add that these photographs were submitted to both 

 Professor Haswell and to Dr. S. J. Johnston, of the Zoological 

 Department, University of Sydney, and both of them pronounced 

 them to be artefacts without any doubt whatever. 



It seems necessary to go into this matter at some length, in 

 order to prevent further speculations as to the function of these 

 fibril-tufts. The fascinating theory that each alveolus might be 

 a single cell, in which special end-oi-gans {i.e., the fibril-tufts) 

 were developed, for the purpose of extracting oxygen from the 

 alveolar contents (water, air, or blood ?j and passing it into the 

 tracheal system, can no longer be entertained. We must look 

 for some simpler, though more prosaic, explanation of the physi- 

 ology of the caudal gills. 



Let us now return to the question of the structure of the 

 alveoli themselves. Here a study of these organs in all positions 

 within the gill, and in all different types of gill available, is 

 essential to aright understanding of their nature. Of particular 

 value is a careful study of those alveoli, few and comparatively 

 large in number, which lie in the rachis of a lamellar gill; since 

 in this case the mind is not confused l^y their immense number, 

 and their relationship with the hypoderrais is easily made 

 apparent. 



In those alveoli which lie closest to the hypoderm-layer, it is 

 easily seen that the wall of the alveolus merges into the inner- 

 most stratum of the hypoderm-layer, as shown in Text-fig. 9. It 

 would appear that the hypoderm-cell is capable of throwing ofi* 

 a portion of its substance in the form of a delicate layer, either 

 from its inner or outer border. The continuous layers so thrown 

 off along the whole hypoderm-layer upon its outer border form 

 the separate sti'ata of which the cuticle is composed. Between 

 the layer last thrown off, and the living hypoderm-cells, one finds 



