156 LIFE BY THE SEASHORE. 



keep down the number of technical terms in this book so 

 far as possible, they have been used whenever clearness and 

 conciseness would be sacrificed by their absence. Among 

 the Crustacea especially, a certain number of such terms 

 seem absolutely necessary, if the relation between the 

 different forms is to be made clear. 



Returning to the study of the crayfish, it is obvious that 

 if the cephalothorax contains thirteen united segments, and 

 the tail six free ones, and each of these segments bears a 

 pair of appendages, then there must be nineteen pairs of 

 appendages, apart from the tail-piece, or telson. These 

 nineteen pairs of appendages are most easily studied by 

 beginning at the posterior end, removing the appendages of 

 one side successively, and laying them out in order. 



In the following list they are for convenience described 

 from before backwards : 



(1) First antennae, or antennules, consisting each of a 

 stalk, or peduncle, and two short whips, or flagella. 



(2) Second antennae, or antennae proper, consisting each 

 of a peduncle, bearing an outer broad flat scale, or squame, 

 and a long inner flagellum. 



(3) The mandibles, hard, toothed plates, close to the 

 mouth. 



(4) First pair of maxittce, or jaws, small, delicate, and 

 probably functionless. 



(5) Second pair of maxillae, also very delicate, but 

 furnished with a plate the baler of much importance 

 in respiration. 



(6, 7, 8) Three pairs of foot-jaws, or maxillipedes, con- 

 sisting of a basal piece and an inner and an outer branch. 

 The inner branch, especially in the third maxillipede, is 

 more or less leg-like (see b in Fig. 50). 



(9) The great forceps, or chelipedes. 



(10, 11, 12, 13) The four pairs of walking legs, all with 

 seven joints. (It is because of the presence of these five 

 pairs of "legs" (appendages 9-13) that the three types are 

 included in the order Decapoda.) 



(14, 15, 16, 17, 18) The small swimmerets, typically 

 consisting of a basal piece and an outer and an inner 

 branch, but the first two pairs are more or less modified in 

 the male. 



