142 ELEMENTARY BIOLOGY. [CHAP. 



smooth facet on the anterior half of the 

 pleuron where it is overlapped by the one in 

 front. 



I. The appendages or swimmerets: one on each side: 

 the structure of each 



a. The short two-jointed basal portion (protopo- 

 dite), consisting of a shorter proximal and a 

 longer distal piece. 



/?. The antero-posteriorly flattened elongated 

 lamellae attached to the distal joint of the 

 protopodite, an inner (endopodite) and outer 

 (exopodite). 



3. The fourth and fifth abdominal segments : closely 

 resembling the third. 



4. The sixth abdominal segment: its modified append- 

 ages. 



a. The protopodite : represented by a single short 

 strong joint. (In the lobster there is an in- 

 complete basal joint.) 



J3. The exopodite and endopodite : wide plates 

 fringed with setse : the exopodite divided into 

 two portions by a transverse joint. 



5. The telson. 



A flattened plate bearing no appendages : sub- 

 divided by a transverse joint (it is undivided in 

 the lobster) : the membranous character of the greater 

 part of the ventral surface of its anterior division. 



The tail-fin ; formed by the telson and the append- 

 ages of the sixth abdominal segment. 



