SECT, vi THE SENSORY ORGANS 87 



(8) Hooked or bent hairs on the first antennae 

 which we homologise with the olfactory hairs of 

 the higher Crustacea (see Fig. 7, p. 34). 



To these may be added : 



(e) Fine sensory hairs thickly covering the inner 

 surfaces of both upper and lower lips. 



The gill, as already stated, has no hairs, since these 

 would hinder the free flow of the respiratory medium. 



Besides these different kinds of hairs, there are 

 stiff denticulate bristles in rows near the bases, on the 

 flat surfaces, of the rhomboidal feelers (the endites), 

 which probably hinder the escape of prey between 

 the limbs (see Fig. 5, p. 23, and p. 46). 



While, perhaps, the finer sensory hairs may be 

 homologised with similar tactile hairs in the Annelida, 

 it is not easy to homologise the more highly developed 

 setae. If any of the original Annelidan setae have 

 been transformed into the hollow Crustacean hairs, 

 the transformation would have to be described some- 

 what as follows. The thickening of the cuticle 

 supplying a firm base for the seta, it would not be 

 necessary for it to sink below the integumental surface. 

 Again, the integument not being liable to be thrown 

 into folds like the soft skin of the Annelida, 

 the seta would not require to be movable ; hence 

 there is no need for it to project from a sac-like 

 group of secreting cells under the cuticle, provided 

 with muscular attachments. The Crustacean seta is 

 a hollow process of the cuticle secreted by a ring of 

 hypodermis cells, through which the nerve runs into 

 the lumen of the seta. At the base of the hair, the 



