306 THE INVERTEBRATA 



joint of the antennule (Fig. 216) and in the Mysidae on the endopodite 

 of the sixth abdominal appendage there is a pit whose wall bears such 

 hairs while the hollow usually contains sand grains (most decapods) or 

 a calcareous body formed by the animal (Mysidae). These organs are 



,ex. 



aes 



op. 



D '«•/ 



Fig. 216. The antennule of Astacus. A, The right antennule, seen from the 

 median side with the basal joint opened and the flagella cut short. B, Two 

 joints of the distal part of the outer flagellum, enlarged, after Huxley. C, The 

 basal joint of the left antennule, from above. D, Two hairs from the stato- 

 cyst. E, An aesthetasc. aes. aesthetasc; en. inner flagellum; ex. outer flagel- 

 lum ; grn. sand grains ; n. nerve ; n.f. nerve fibre ; op. opening of statocyst, over- 

 hung by a fringe of hairs ; stc. statocyst. 



statocysts for the sense of balance. Olfactory hairs or aesthetascs 

 (Fig. 216 B, E) with delicate cuticle stand on most antennules and on 

 many antennae. A pair of groups of cells, sometimes surmounted by 



