NATURAL HISTORY OF AMERICAN LOBSTER. 249 



THE AUMENTARY TRACT. 



The alimentary tract (pi. xxxiir) , extending from mouth to anus, consists of three parts, 

 which are quite distinct in origin, namely: (i) The foregut (stomodaeum of the embryo), 

 formed by a tubular invagination of ectodermic epithelium; this remains distinct until 

 late in embryonic life, and gives rise to the epithelial lining of the esophagus and grinding 

 stomach; (2) the midgut (mesenteron of the embryo), lined with endodermic epithe- 

 lium, and formed by the walling in of the great mass of the yolk by endodermic cells; 

 paired outgrowths or folds of the endodermic sac arise early in embryonic life and eventu- 

 ally form the liver of the adult; aside from the liver or gastric glands, the mesenteron 

 appears to take no part in the formation of the alimentary tract; (3) the hindgut 

 (proctodaeum of the embryo), formed by a solid ingrowth of ectodermic epithelium 

 which subsequently becomes hollowed out, its walls merging with those of the mesen- 

 teron; it gives rise to the lining of the intestine and caecum. 



The foregut and hindgut, being infolded parts of the outer surface of the body, are 

 covered with a cuticle which is continuous with the chitinous exoskeleton, and is cast 

 off in the molt. 



THE GRINDING STOMACH. 



The higher Crustacea are the only animals which grind the food after it reaches 

 the stomach as well as before it enters the mouth. Granivorous birds swallow their 

 food whole, and with the aid of gravel stones or other hard bodies pulverize it in a 

 muscular gizzard; in a number of gasteropod moUusks analogous organs occur, but the 

 stomach mill of a decapod crustacean is a much more complicated machine. 



When a bit of fish or clam is ofifered to a hungry lobster, it seizes the food with the 

 claws of the slender forward legs and passes it up to the mouth, where it is held by 

 the large maxillipeds. The cutting teeth and spines of the mouth parts, especially the 

 maxillae and mandibles, are successively brought to bear upon it, and chop it into 

 mince-meat, while it slowly enters the mouth in a stream of fine particles. 



The stomach of the lobster is truly a complicated mechanism, and could not be 

 fully described without entering into great detail. In the brief account which follows 

 I shall rely mainly upon a study of this subject by Williams {sjg), which is by far the 

 best that has appeared. 



The stomach sac (pi. xxxiii and xxxiv) serves for storing, grinding, sorting, and 

 straining the food, as well as for deUvering the finest particles in liquid streams along 

 definite channels to the intestine and to ducts of the liver; for, as Jordan has shown, the 

 huge gastric glands serve also for the direct digestion and absorption of food. Further, 

 the coarser particles of the food may be sent to mill time and again to be reground, 

 while the indigestible parts are regurgitated. Again, it should be added that newly 

 molted lobsters instinctively devour their own cast, and I have found soft lobsters 

 with their stomachs stuffed full of the shells of moUusks and other calcareous frag- 

 ments (see J49, p. 89) , actions which point clearly to the need of the animals at such times 

 to obtain a supply of lime as quickly as possible. 



