130 The American Naturalist. [February, 
tions: In all metameric animals one or more pairs of nephri- 
dia serve as genital ducts, and no case is known of the forma- 
tion of new outlets. The genital ducts are so related to the 
gonads and these latter to the ccelom, at least in the Decapods 
(cf. Weldon, ’89, 91) that we must regard the genital epithe 
lium as cœlomic, and the ducts as ventral diverticula of the 
same space. 
The salivary glands afford some difficulties, for they occur 
in most “ Tracheates,” and are usually stated to be absent from 
the “ Branchiates.” This apparent difference between the two 
groups is possibly to be explained by the different method of 
life—aquatic in the latter, terrestrial in the former. It is, 
however, to be noted that salivary glands have been recognized 
in Astacus (cf. Lang, ’89, p. 344), while renewed studies must 
be made of the so-called salivary glands of the Arachnida be- 
fore we are certain of their homology with those of the Hexa- 
pods. Several organs which have been called salivary glands 
among the spiders and their allies have been shown to be 
coxal glands (i.e. nephridia) or poison glands, and it is possi- 
ble that all of these organs may have different homologies than 
those indicated by the name usually applied to them. 
A group of structures which cannot, as yet, be discussed, 18 
that of the embryonic membranes. In the Scorpions as in the 
Hexapods, the embryo develops those as yet unexplained foetal 
membranes which so closely simulate those of the higher ve 
tebrates. It may be that here, ‘as in other places, we have 
similar but not identical organs. The accounts of their devel 
opment in the Arachnids by Metschnikoff, Kowalevsky and 
Schulgin, and Laurie differ considerably, and, until we know 
something of the ancestry and real meaning of the structures 
here that the structures described by Bruce (’87) as occurritg 
in the spiders are, in all probability, not amnion and “a : 
but either invaginations in connection with the brain oT™™ 
inpushing to form the médian eye. 
