The Crayfish. 



39 



and a long, many-jointed flagellum.' The scale is believed to 

 represent the exopodite, and the flagellum the endopodite. 



Finally, the antennules have the form illustrated in Fig. 20, B. 

 Each is formed of a basal piece consisting of three joints, which 

 bears distally two approximately equally sized flagella. It is 

 tempting to see here also a biramons limb of the typical 

 character. But it is not clear that this comparison is justifi- 

 able, for it will be observed that the two flagella spring from 

 the third joint, while in all other cases the endopodite and 



Fig. 20. ■ — Head appendages of Crayfish. (After Huxley.) 



A, eye-stalk ; B, antennule ; c, part of antenna ; c, surface of eye ; 

 ex.pt exopodite ; ^.^, orifice of green gland. 



exopodite arise from a protopodite consisting of two joints. 

 More probably, therefore, the bifid character of the terminal 

 part of the antennules has nothing to do with the biramous 

 limb, but is a simple fission or a secondarily added appendix. 

 Some naturalists have endeavoured to show that the eye, or 

 rather the eye-stalk, is a rudimentary appendage, consisting in 

 this event of the protopodite only. The stalk is certainly 

 movable ; but as the structure arises from a different part of 



' The use of the term "flagellum" for a piece composed of numerous 

 minute joints will not be confounded with the long vibratile cilium, which, 

 when single or in groups of very few, in Protozoa and other animals bears 

 the same name. 



