14 CRUSTACEA 



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 Mysis and 

 JSfebalia 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 eudoderm 

 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-(fut 

 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 tlie 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 prol)al)ly 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 w^hich 

 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, e.g. CaUlanassri. suh- 

 terranea, the length of the mid-gut between the anterior and 

 posterior caeca is very long ; in Carcinus maenas it is consider- 



