xx. CRAYFISHES. 293 



shaped; but if he be a female (excuse my mixed genders) t 

 she will have the first of these pairs smaller than the rest, 

 or even wanting altogether. Behind the hinder edge of 

 the hinder bar are the side plates of the tail flap. They, 

 like the swimmerets, are appendages of the body, but 

 they are large and flattened, and developed for the special 

 purpose of serving as a tail fin. 



The legs are now seen to be many-jointed appendages. 

 Examine the joints, as you bend one of the legs, to see how 

 the various hinges work in different planes. Each is 

 capable of free movement backwards and forwards in one 

 direction, but, like your own elbow-joint, in this direction 

 only. The successive hinges are, however, nearly at right 

 angles to each other, and so the limb, as a whole, has 

 tolerably free play. Notice the large pincers of the claw. 

 If, when the crayfish was alive, he succeeded in giving you 

 a nip, you will doubtless wish to know how he did it. I 

 am not going to tell you ; but I will show you how to see 

 some part of the mechanism for yourself. Cut off the claw 

 at the end of the appendage, and observe the larger and 

 the smaller joint. With a strong pair of scissors remove 

 the shell from one side of the swollen part of the larger 

 joint. There are the white muscles which, by their 

 contraction, moved the smaller joint when the crayfish 

 was alive. If you open and shut this joint you will see 

 that the muscles are disturbed. And if you scrape away 

 some of the muscles, you will find embedded in them two 

 flat plates, which are connected with the small joint. Each 

 of these is attached to, and pulled by, a separate muscle, 

 in which it is embedded. The rest I leave you to find 

 out for yourself. Note how the small joint is hinged ; and 

 observe the effects of pulling first on one of the flat plates 

 and then on the other. If you doubt whether the white 

 muscles you have seen are large enough to close the 



