I4 CRUSTACEA CHAP: 
any rate is a composite structure, formed by an ectodermal 
ingrowth which meets a mesodermal strand, and from the latter 
are produced the end-sac and perhaps the tubular excretory 
portions of the gland with their derivatives. 
With regard to the possible metameric repetition of the 
renal organs, it is of interest to note that by feeding M/ysis and 
Nebalia on carmine, excretory glands of a simple character were 
observed by Metschnikoff situated at the bases of the thoracic 
limbs. 
The alimentary canal of the Crustacea is a straight tube 
composed of three parts—a mid-gut derived from the endoderm 
of the embryo, and a fore- and hind-gut formed by ectodermal 
invaginations in the embryo which ‘push into and fuse with the 
endodermal canal. The regions of the fore- and hind-gut can 
be recognised in the adult by the fact. of their being lined with 
the chitinous investment which is continued over the external 
surface of the body forming the hard exoskeleton, while the 
mid-gut is naked. The chitinous lining of fore- and hind-gut 
is shed whenever the animal moults. In the Malacostraca, in 
which a complicated “ gastric mill” may be present, the chitinous 
lining of this part of the gut is thrown into ridges bearing 
teeth, and this stomach in the crabs and lobsters reaches a high 
degree of complication and materially assists the mastication of 
the food. The gut is furnished with a number of secretory and 
metabolic glands; the so-called liver, which is probably a hepato- 
pancreas, opening into the anterior end of the mid-gut, is directed 
forwards in most Entomostraca and backwards in the Malacostraca, 
in the Decapoda developing into a complicated branching organ 
which fills a large part of the thorax. In the Decapoda peculiar 
vermiform caeca of doubtful function are present, a pair of which 
open into the gut anteriorly where fore- passes into mid-gut, 
and a single asymmetrically placed caecum opens posteriorly into 
the alimentary tract where mid- passes into hind-gut. 
The disposition of these caeca, marking as they do the 
morphological position of fore-, mid-, and hind-gut, is of peculiar 
interest owing to the variations exhibited. From some un- 
published drawings of Mr. E. H. Schuster, which he kindly lent 
me, it appears that in certain Decapods, eg. Callianassa sub- 
terranea, the length of the mid-gut between the anterior and 
posterior caeca is very long; in Carcinus maenas it is consider- 
