II.] THE CRAYFISH AND LOBSTER. 195 



and Crayfish, two lateral spines; in the latter animal 

 it is segmented, at the point of origin of these, into 

 two portions — an anterior one, the tergal surface of 

 which remains largely chitinous, and a terminal post- 

 anal one, which is completely calcified. 



The perianal area 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 poinis of 

 attachment to the axial skeleton. 



Examine under water, posterior face upper- 

 most. 



a. The third ahdouiinal appendage. Its general charac- 

 ters have been described (i. /^); examine in 

 detail — 



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

 shorter proximal hip-segment or coxopodite^ and 

 a long distal one {basipodite) which forms a base 

 of articulation for the ^xopodite and endo- 

 podite. 



yS. Its endopodite (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 /?. It also 

 is subdivided into a single basal and a terminal 



# 13—2 



