CRUSTACEA. 329 



simple intestinal tube, which passes in a direct line from the sto- 

 mach to the last segment of the abdomen, where it terminates. 



The description of these parts, as they exist in the lobster, 

 will give the reader a sufficiently correct idea of their general 

 disposition and structure ; nor are we acquainted with any class 

 of animals in which so little variety in the conformation of this 

 portion of the system is to be met with. 



The oesophagus is covered at its origin by the several pairs of 

 foot-jaws already alluded to ; the most internal of which forms 

 a decided cutting apparatus, resembling a pair of strong shears, 

 while the rest are only instruments of prehension, or, perhaps, of 

 sensation also. From the mouth, the oesophagus runs directly up- 

 wards to the stomach, which is a considerable viscus (Jig. 157, a), 

 a large portion of it being situated in that region of the cephalo- 

 thorax which we should be tempted to consider as the head of the 

 animal. The pyloric extremity of the stomach is strengthened 

 with a curious frame-work 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 bruis- 

 ing the food before it is admitted into the intestine. 



The intestine itself (, b, I) 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 ori- 

 fice closed by a sphincter muscle. 



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

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

 tween them the pyloric portion of the stomach, and a third part of 

 the length of the intestine. When unravelled, the minute struc- 

 ture of the liver exhibits an immense assemblage of secerning caeca 

 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 (d), which pours the bile de- 

 rived 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 so- 

 litary long and slender caecum enters the intestine, but the nature 

 of the secretion furnished by this organ is unknown. 



(368.) Before tracing the course of the circulation in the Crus- 

 tacea, it will be necessary to consider the character of the apparatus 



