A LOBSTER OR A CRAYFISH. 55 



h) The thoracic appendages. The posterior half of the cephalothorax 

 bears five pairs of walking legs, the first pair, called chelipeds, 

 terminating in large pincher claws, the chelae. These are for- 

 midable organs of offense and defense and are also useful in 

 securing food. Are the two chelae alike in size and shape ? The 

 last four pairs of legs are called periopods. How many pairs 

 terminate in chelae ? On the ventral surf ace of the basal segment in 

 the last pair look for the genital pores, if the specimen is a male. 

 If a female these will be found similarly located on the second 

 pair from the last. In front of the chelipeds are three pairs 

 of maxillipeds, which are classed with the mouth parts. Re- 

 move these carefully from the right side, beginning with the 

 third pair. 



i) Appendages of the head. Preceding the maxillipeds are two more 

 pairs of mouth parts, the maxillae. To the second of these is 

 attached the gill scoop, an organ whose function is to scoop the 

 water out of the front part of the gill chamber, thus allowing a 

 fresh current to be constantly drawn in at the free ventral margin 

 of the gill cover. Remove the right maxillae. This will expose 

 the mandibles, the real jaws of the animal. To each is attached 

 a jointed palpus, and the two are bordered posteriorly by a pair of 

 small flaps, the paragnatha, which are not considered true ap- 

 pendages. Between the mandibles is the mouth, bounded in 

 front by the labrum. Finally we come to the antennae and the 

 branched antennules, the latter the smaller and more anterior. 

 On the ventral side of the basal joint of the former notice the 

 opening of the green gland, an organ to be observed later. On 

 the dorsal surface of the corresponding joint of the antennules is a 

 slit or depression with a minute opening into a sack filled with a 

 liquid containing fine sand grains. This structure has been 

 termed the otocyst and to it was assigned the function of hearing. 

 Many zoologists, however, believe it to be a balancing organ. In 

 the fine, hair-like projections of the antennae is believed to be 

 located the sense of smell. The otocyst will not be studied in this 

 exercise. 



