BY R. J. TILLYARt). 5? 



form, and would settle down near the alveolar wall in the form 

 of an amoebocyte. The absence of miocytes within the alveoli 

 simply proves the absence of any definite circulation of blood 

 within them. 



The Internal Lamina. 



In all caudal gills which J have examined, except the Leatid 

 and Reduced Types, the alveolar mesh work is seen to be 

 strengthened, along certain definite zones, by a series of thicker 

 ingrowths from the hypoderm-layer. These ingrowths were first 

 noticed in the larv a of Faeudophaa by Ris (28), who termed them 

 "lamellen." As, however, the term "lamella'' has already been 

 employed more generally for the flat, blade-like gills of Lestidct- 

 and Ayrionidce, I propose to substitute the term internal lamina 

 for these ingrowths, in order to avoid confusion. 



In all forms of caudal gill, the internal lamina? occur chiefly 

 along two well-marked longitudinal zones. One of these zones 

 would be represented, in all median gills except those of the 

 Horizontal Lamellar Type, by a plane perpendicular to the 

 plane of symmetry of the gill, at a level slightly dorsad from the 

 dorsal blood-canal. Dorsad from the first plane, and ventrad 

 from the second, more scattered internal laminte are developed: 

 but I have not found any general development of lamina? in the 

 more central area between the two blood-canals. Only in the 

 Agrionid gills of the Vertical Lamellar Type have I found 

 laminae developed just ventrad from the dorsal blood-canal, or 

 dorsad from the ventral blood- canal. 



In lateral gills of the Vertical Lamellar Type, the zones of 

 laminae agree with those of the median gill. But in lateral gills 

 of the Triquetial Type, the zones may best be described as being 

 developed along two planes which cut off, in any given cross- 

 section, the upper and inner corners of the triangle representing 

 the section. These two planes are therefore not parallel, but 

 somewhat inclined to one another inwards. The same holds 

 good in the lateral gills of the JSaccoid Type. The positions of 

 the two principal zones of laminae are in all cases clearly repre- 

 sented in the series of diagrammatic sections in Text-figs. 11, 17, 

 22 {dl, vl). 



