CRUSTACEANS 



407 



rhe remainder may be taken in connection with the various 

 systems of organs which are concerned, and which will be con- 

 sidered seriatim. 



Digestive Organs {^g. 249). — These consist of (i) a symmetrical 

 food-tube or gut, which runs back from the oval mouth situated 

 on the under side of the head to the vent placed below the telson, 

 and (2) of digestive glands. It will be noted that, as in Arthropods 

 generally, the jaws are modified limbs, which work against one 

 another from side to side and are entirely outside the mouth, 



OPHTHALMIC 

 ARTERY 



Fig. 



AB 7; ITELSON) 



249. — Side-dissection of Lobster {Homarus vulgaris)^ reduced, ab i-ab 7, Abdominal segments. 



which is bounded by upper and lower lips. It is found convenient 

 to speak of the gut as being divided into three sections — a large 

 fore-gut, a very small mid-gut into which the digestive glands 

 open, and a long hind -gut. The distinction between these sec- 

 tions is based on the mode of development, for the first and last 

 of them begin as pits which extend farther and farther inwards 

 till they join the developing mid-gut, and form with it a continuous 

 tube. It is not therefore surprising to find that the fore- and 

 hind-guts, since they are formed by inpushing of the body-wall 

 (see p. 261), are lined by a firm horny layer continuous with 

 the exoskeleton, and that this layer is shed and replaced every 

 time the animal moults. The fore-gut consists of a short gullet 

 dilating into a very large stomach, divided into a cardiac section 

 in front and a pyloric part behind. The stomach is said to be 

 a " masticatory " one, for it contains a chewing apparatus or 

 gastric mill, consisting of a number of hard pieces formed by 

 thickening and calcification of the horny lining. These pieces 



