ALIMENTARY AND RESPIRATORY GROANS. 395 



From the mouth, the oesophagus runs directly upwards to the stomach, 

 which is a considerable viscus (fig. 201, a), a large portion of it being 

 situated in that region of the cephalothorax which we should be tempted 

 to consider as the head of the animal. The pyloric extremity of ,the 

 stomach is strengthened with a curious framework of calcareous pieces 

 imbedded in its walls, and so disposed as to support three large teeth 

 placed near the orifice of the pylorus ; and being moved by strong 

 muscles, teeth so disposed no doubt form an efficient apparatus for 

 bruising the food before it is admitted into the intestine. 



(1009.) The intestine itself (b b 6) runs in a direct course to the tail, 

 imbedded between the two great lateral muscular masses that move the 

 abdominal segments, and terminates upon the ventral surface of the 

 central lamella of the terminal fin, in a rounded orifice closed by a 

 sphincter muscle. 



(1010.) The liver (c c c), one half of which has been removed in the 

 engraving, consists of two- large symmetrical masses, enclosing between 

 them the pyloric portion of the stomach and a third part of the length 

 of the intestine. When unravelled, the minute structure of the liver 

 exhibits an immense assemblage of secerning ca3ca agglomerated into 

 clusters, from each of which a duct emanates ; and the continued union 

 of the ducts so formed ultimately gives origin to the common hepatic 

 canal (cT), which pours the bile derived from that division of the liver 

 to which it belongs into the intestine, at a very short distance from its 

 commencement at the pylorus. A little below the insertion of the two 

 bile-ducts, a solitary long and slender caecum enters the intestine ; but 

 the nature of the secretion furnished by this organ is unknown. 



(1011.) In the highest Crustacea, as the Decapoda, in which legs of 

 an ambulatory character become such important locomotive agents, it 

 is principally to the origins of these legs that we find the breathing 

 apparatus appended ; and their active motion will consequently power- 

 fully contribute to the complete aeration of the blood. But let us first 

 examine the structure of the branchiae themselves in this highly- orga- 

 nized division, and subsequently we will speak of their arrangement and 

 connexions. 



(1012.) In the Lobster, and many other Macroura, the branchiae 

 (fig. 204, m m), are pyramidal tufts, consisting of a central stem covered 

 over with vascular filaments disposed perpendicularly to its axis, in such 

 a manner that each of these organs, when detached, resembles in some 

 degree a'small brush : on cutting the stem across, it is found to enclose 

 an artery and a vein, from which innumerable branches are given off to 

 the horizontal filaments; so that the latter constitute a respiratory 

 surface of great extent, which is most freely exposed to the surround- 

 ing medium. 



(1013.) In the Crabs and Anomoura the structure of the branchiae 

 is somewhat different ; for in these divisions the cylindrical filaments are 



