Not very much is known of the development of these various structures. Some authors have considered that both antennary and maxillary glands are developed in the embryo from ectodermal inpushings, but the more recent observations of Waite[11] on _Homarus americanus_ indicate that the antennary gland at 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 _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 hepatopancreas, 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.