II.] THE CRAYFISH AND LOBSTER. 2OI 



The distal segment of the protopodite (basipo- 

 dite) is subdivided into two ; the outer segment is 

 squamous in the Lobster. 



m. The antennule. Its large three-jointed basal seg- 

 ment (protopodite), bearing a pair of multi-articulate 

 filaments (endopodite and exopodite) : the opening of 

 the auditory organ, in the midst of a tuft of setae 

 on the upper surface of the basal joint. The exo- 

 podite, the longer of the two jointed filaments, is 

 carried erect during life (it bears sensory setae. 

 Cf. Sects. A. f. e. and K. 2). 



n. The ophthalmite or eyestalk. A short two-jointed 

 structure which appears to represent the protopo- 

 dite of an appendage. (Cf. Sect. K. 3 c.) 



5. Remove that half of the body from which the ap- 

 pendages have been dissected, thus reducing the 

 whole to the condition of longitudinal vertical section; 

 boil for a few minutes in weak solution of Caustic 

 Potash, pick away the soft parts and examine from 

 within. 



a. The inter-articular membranes ; flexible, persistently 

 uncalcified, portions of the exoskeleton. Note that 

 (in the Crayfish) between the two last thoracic 

 sterna. (Cf. Sect. B. 2 6.) 



b. The endophragmal system. Each set of endoster- 

 nites and endopleurites of which it is composed (cf. 

 Sect. B. 2 d.} is seen to arise in the same plane ; 

 each endosternite slopes forwardly, carrying with it 

 the anterior limb of its corresponding endotergite, 

 the posterior limb of the one in front passing back to 

 meet it. The points of apposition of the successive 

 sets of elements alternate with those of their origin. 



