J 34 



Structure of a gill (tigs. 05, GO, 67). 



Each gill is of the phyllo-branchiate type. With the 

 exception of the third gill, they are all pyramidal in 

 shape, their apices pointing upward (with the exception of 

 the first gill, in which the apex points backward). Along 

 the outer side of each gill runs the afferent branchial 

 vessel, and the efferent vessel is situated on the inner side. 

 The gill is composed of numerous lamellae, which have 

 the appearance of the leaves of a book. Each lamella is 

 covered with a thin layer of chitin. This layer is also 

 continued on the outside of the afferent and efferent 

 vessels. The gills, therefore, are covered by part of the 

 general chitinous exoskeleton, and at ecdysis this outer 

 chitinous layer is cast with the remainder of the 

 exoskeleton. In transverse section each gill is triangular 

 (iig. (J5). The efferent vessel is situated at the apex of 

 the triangle, and the afferent vessel lies in the middle of 

 the base of the triangle. Stretching across from the 

 afferent to the efferent vessels is the branchial septum 

 (i.b.s.), which separates the anterior from the posterior 

 lamellae. 



In the branchial septum between the afferent and 



efferent vessels transverse sections reveal the presence of 



scattered cells, generally having brown contents (fig. 07, 



br. c). These are excretory cells, and together constitute 



the branchial excretory organ (see section on Excretory 



System). (Atenot* found that when a crab is injected 



with ammonium carminate or methylene green these 



substances are taken up by the excretory cells of the 



branchial septum. These cells, therefore, have the same 



reaction as the end-sac epithelium. 



I have not been able to find any trace of the branchial 



glands discovered by Allent in Palaemonetes. Cuenot, 



* Cuenot. Arch, de Biol, T. XIII, p. 245. 

 t Allen. Q.J.M.S., Vol. XXX IV, p. 75. 



