14 CRUSTACEA CHAP. 



uny rate is u composite structure, formed by an ectodermal 

 ingrowth which meets a mesodermal strand, and from the latter 



O 



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 My sis 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 chitiuous 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 Decapod; i 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, r.y. CaUianassa sub- 

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

 posterior caeca i- very long; in Cni'dnus maenas it is consider- 



