II.] THE CRAYFISH AND LOBSTER. 195 



and Crayfish, two lateral spines (/ureal processes) ; in 

 the latter animal it is segmented, at the point of 

 origin of these, into two portions — an anterior peri- 

 anal one, the tergal surface of which remains largely 

 chitinous, and a terminal post-anal one, which is 

 completely calcified (cf. p. 1 76). 



The circum-anal cuticle is calcified on opposite 

 sides, to form a couple of anal plates ; _they are most 

 marked in the Lobster. 



Note the delicate setce, which fringe the free border 

 of the telson. 



4. The appendages. 



Remove the entire set from one side of the body in 

 the order enumerated below, cutting through the 

 inter-articular membranes close to their points of 

 attachment to the axial skeleton. 



Examine under water, posterior face upper- 

 most. 



a. The third abdominal appendage. Its general charac- 

 ters have been described (1. b.); examine in 

 detail — 

 a. Its protopodite ; made up of two segments — a 

 shorter proximal hip-segment or coxopofite, and 

 a long distal one (basipodite) which forms a base 

 of articulation for the exopodite and endo- 

 podite. 



j3. Its endopodiie (the longer of the two terminal 

 portions). It is subdivided into two segments 

 of equal length — the basal one is continuously 

 calcified, the terminal one is multi-articulate. 



y. Its exopodite; relatively shorter than ft. It also 

 is subdivided into a single basal and a terminal 



13—2 



