88 THE APODID^ parti 



nerve swells into a ganglion. In the feathered hairs, 

 a fibre runs into each barb, and the ganglion is a 

 regular group of cells (see Fig. 31, p. 131). This 

 nerve may well be the nerve which originally supplied 

 the setiparous gland of the Annelid, and the ring of 

 secreting cells all that remains of the sac itself. 



On each side of the under lip is a straight longi- 

 tudinal row of sensory papilla. As these are some- 

 times found thickly clogged with particles of food, 

 they probably border the channel leading into the 

 oesophagus on each side, to hinder juices, &c., from 

 escaping laterally under the mandibles. 



THE EYES. 



We are here brought face to face with a problem 

 of no small difficulty. It is clear that if our theory 

 is true, we have to attempt to explain the develop- 

 ment of the eye of Apus, i.e., the development of the 

 typical Crustacean eye from that of the Annelids. It 

 is hardly necessary to dwell upon the difficulty of 

 such a task, since it is but too well known that the 

 last word has not been said as to the actual structure 

 of either the Crustacean or the Annelidan eye. Still 

 we cannot turn from the attempt, especially as we 

 hope to be able to show that if we do not go 

 into too many details, and at the same time keep con- 

 stantly in mind the effect which the development of 

 a thick chitinous cuticle would naturally have upon 

 the hypodermal eye-spots, it is possible to sketch a 

 fairly probable origin of the Arthropodan eye. 



