408 EDGAR J. ALLEN. 



the end-sac is not in free communication with that of the tube, 

 the entrance from the one to the other being guarded by 

 certain elongated cells of the end-sac which project into the 

 lumen of the tube. Two such cells appear in each section. 

 This arrangement of cells is invariably found at the point 

 where the end-sac joins the tube, and appears to constitute a 

 valve which would allow fluid to pass out of the end-sac, but 

 would prevent it from returning in the opposite direction. 

 The structure of the walls of the renal tube of the shell-gland 

 is the same as that of the corresponding parts of the green 

 gland. 



In his account of the development of Eriphya spini- 

 frons, Lebedinski (No. 16) describes a pair of " segmental 

 organs " which develop as evaginations of the somatopleure, 

 and which, according to him, communicate directly with the 

 body-cavity, and open at the bases of the first pair of raaxilli- 

 pedes. Korschelt and Heider (No. 14) have already suggested 

 that this organ may be the shell-gland. If this suggestion be 

 correct, Lebedinski must either have mistaken the appendage 

 on which the gland opens and overlooked the end-sac, or the 

 arrangement in the species examined by him must vary from 

 that which exists in Palsemonetes and Palsemon, and 

 which upon theoretical grounds appears the more probable. 



It is a fact of interest, and one which is of some importance 

 in a consideration of the phylogeny of tlie various groups of 

 Crustacea, that in the Entomostraca (B ranch i pus, Claus, 

 No. 6; Cetochilus, Grobben, No. 11) the green gland is the 

 first of the two pairs of nephridia to become functional, the 

 full development and activity of the shell-gland being deferred 

 to the post-larval period when the green gland has almost 

 disappeared, whilst among the Decapods the periods of deve- 

 lopment of the two glands are reversed. 



The shell-gland is, I believe, functional in Palsemonetes 

 even before hatching, as I have observed it in sections of 

 embryos, some time before this event, with a well-developed 

 lumen, with the characteristic striation of the protoplasm, 

 with a definite external opening, and with a mass of yellowish 



