No. 2.] THE EMBRYOLOGY OF LIMULUS. 24I 



ever, nephridia occurring either in the young or the adult of 

 other Arthropods, and hence a more accurate review of our 

 knowledge becomes necessary. In Peripatus the investigations 

 of von Kennell ('84), and especially of Sedgwick ('88), have 

 shown that in each somite, except the posterior one, the coelom 

 on either side divides into dorsal, lateral and ventral moities; 

 the dorsal becomes converted into the gonad, while the ventral 

 portion becomes converted one part into the nephridium and the 

 lateral into the funnel and end sac. The connection between 

 the dorsal and ventral portions of the coelom persists in the 

 posterior somite, and from the cavity thus formed the genital 

 ducts are developed, in other words, the posterior nephridia 

 of Peripatus become modified for reproductive ducts. In 

 the Hexapods Heymons ('90), Graber and Cholodkowsky ('9l)^ 

 have described a similar division of the coelom into three por- 

 tions, the gonad developing in connection with the dorsal 

 portion; the formation of genital ducts, much as in Peripatus; 

 and the development of the third division into a temporary 

 structure to be regarded as the homologue of the nephridium 

 of Peripatus, and which later disappears. In the Crustacea 

 nephridial structures also occur. Our knowledge of them and 

 of their relations to the coelom are most detailed in Decapod. 

 Here Weldon ('89, '9l) has described a large coelomic dorsal 

 sac, extending back to the heart and the gonads and connected 

 ventrally with the green (antennal) gland, the character of 

 which as a nephridium is thus placed beyond question.^ The 

 position of the so-called "shell gland" is less certain, though 

 all evidence goes to show that this is also to be regarded neph- 

 ridial. As I pointed out several years ago this organ, opening 

 in the Crustacea at the base of the second maxilla is apparently 

 exactly homologous with the coxal gland of Limulus. Al- 

 though recent researches l^oide supra) have changed our views 



1 There is not full agreement between these authors as to the details of the 

 process. 



2 Both Grobben ('79) and myself ('89) have shown that the green gland of the 

 decapod is mesodermal. Richenbach in his first paper upon Astacus ('77) stated 

 that it was derived from the ectoderm. Although he was corrected in this by 

 Grobben he reiterates his account in his later paper on the crayfish ('86) and 

 ignores Grobben's correction. 



