NATURAI. history OF AMERICAN LOBSTER. 
249 
THE ALIMENTARY TRACT. 
The alimentary tract (pi. xxxiii) , extending from mouth to anus, consists of three parts, 
which are quite distinct in origin, namely: (i) The foregut (stomodseum 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 ca;cum. 
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 mollusks 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 offered 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 (279), which is by far the 
best that has appeared. 
The stomach sac (pi. xxxiri and xxxiv) serves for storing, grinding, sorting, and 
straining the food, as well as for delivering 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 mollusks 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. 
