SECT. XII RELATION OF APUS TO CRUSTACEA 201 



The arterial envelope surrounding the nervous 

 system will be referred to later in the paragraph on 

 the circulatory system. 



The Sensory Organs. — We have already (Fig. 22, 

 p. 91) described and figured the eyes of LImulus in 

 order to explain the origin of the typical Arthropo- 

 dan eye of Apus from the Annelidan eye-spots. It 

 is of no small interest to remark that we had selected 

 the eye of Limulus as a guide towards explaining 

 the origin of the Crustacean eye at the very outset 

 of our investigation, when w^e were entirely occupied 

 in attempting to deduce Apus from an Annelid, and 

 long before it occurred to us that Limulus was 

 probably related to Apus. The establishment of 

 the relationship between the two thus lends con- 

 siderable support to the theory put forward in Part I. 

 as to the possible development of the Arthropodan 

 eye out of an Annelidan hypodermal eye-spot by 

 the thickening of the cuticle. If this deduction is 

 correct, then the eye of Limulus is more primitive 

 than that of Apus. This indeed we might expect 

 from the manner of life of the two, the free- 

 swimming form naturally having the more perfect 

 visual organs, while Limulus, w^hich burrows in mud 

 or sand and lives practically under a roof, has eyes 

 comparatively weakly developed 



The wandering of the eyes from the ventral surface 

 on to the dorsal, w^hich we found indicated by the 

 bend of the cerebro-oesophageal commissures in Apus, 

 is here shown in an equally interesting way by the 

 upward, forward, and outward bend of the long optic 



