A CRAYFISH OR A LOBSTER 23 



Study the ventral side of the cephalo thorax. The most impor- 

 tant organs here are the appendages. At the anterior end of 

 the body are the two pairs of antennae, the longer pair being the 

 second. On the lower surface of the basal joint of each of the 

 latter is an opening ; these are the external openings of the kid- 

 neys, or green glands. Back of the antennae is the mouth. It is 

 bounded in front by a liplike structure called the labrum, and at 

 the sides by the strong mandibles. Press the mandibles aside and 

 pass a probe into the mouth. Between the mouth and the large 

 claws are five pairs of appendages which assist in the act of eating ; 

 they are two pairs of delicate leaf like maxillae, just back of the 

 mouth, and three pairs of larger maxillipeds, back of them. They 

 are best identified by beginning with the hinder pair of maxilli- 

 peds, which is just in front of the large claws, and working for- 

 ward, placing a needle or knife between the appendages as they 

 are identified. Back of the maxillipeds come the large grasping 

 claws, or chelipeds, which form the principal weapons of offense 

 and defense of the animal, and in the largest lobsters are powerful 

 enough to crush a man's arm. Note the difference between the 

 right and the left claw, if any. Back of the chelipeds are four 

 pairs of walking legs. In the male animal the paired external 

 openings of the genital organs are at the base of the last pair of 

 walking legs, in the female at the base of the third pair from the 

 last. Find them. 



The Abdomen. The seven somites forming this body division 

 are all free and jointed with one another. Note the difference in 

 the thickness of the cuticula on the dorsal and the ventral sur- 

 faces, also its thinness at the joints. The appendages on the abdo- 

 men have various uses. They probably have a general respiratory 

 function. In the male the first two pairs are functional in pairing ; 

 in the female the first five pairs hold the eggs from the time they 

 are laid until the young are hatched. The last pair in both sexes 

 is large and broad and with the end segment forms the swimming 

 fin. The end segment is called the telson ; it bears no appendages ; 

 the anal opening is in its ventral side. 



The natural color of the animal is usually a greenish black, but 

 hot water or a preservative turns it red. 



