LESSON 36.] NUTRITION IN THE CEUSTACEA. 



127 



FIG. 



The whole of this curious gastric apparatus (the three large teeth) 

 constitute what is popularly called " the lady, in the head of the 

 Lobster," and, when inverted, the rounded tooth personates " the 

 lady," who is supported by the two lateral grinding teeth. 



583. The alimentary canal passes, without convolutions, through 

 the long axis of the body, and opens by distinct apertures at each 

 extremity. The mucous coat sometimes forms folds in the oesopha- 

 gus and stomach ; the muscular layer is strongest at the orifices of 

 the stomach, and there is no mesentery in the abdomen. The py- 

 loric extremity of the stomach receives on each side a short and wide 

 duct from the liver, but this part of the subject will be best under- 

 stood by reference to the accompanying figure. 



A longitudinal section of a Lobster (Astacus marinus) is given 

 in Fig. 209. The under jaws (a), with their feelers, are placed on 

 the inferior aspect of the head, between the large claws and the two 

 pairs of antennae. 



The short oesophagus opens into the 

 stomach (5), which is surrounded by the 

 numerous lobes of the liver (n, n). The 

 intestine (c) receives at its commence- 

 ment the two hepatic ducts, and passes 

 beneath the heart (e) ; the posterior aor- 

 ta (h) follows nearly a straight course to 

 the vent, which is situate below the last 

 segment of the body (d). 



584. The anterior aortic vessel (/), 

 and the pulmonary vein (</), are both 

 distinctly seen. 



585. In addition to the vascular sys- 

 tem already described, consisting of the 

 heart and arteries, there is yet another 

 portion of the utmost importance, name- 

 ly, venous sinuses. 



586. Instead of veins, the blood is 

 returned to the heart by these struc- 

 tures, which are large, loose sacs, the 

 coat of which is of delicate transparen- 

 cy. The heart itself is enclosed in a ve- 

 nous sinus, and lies literally bathed in 

 blood. These sinuses communicate 



freely with each other, and with the arterial system. The best way 



Section of Lobster. 



