THE EYES OF PERIPATUS 



113 



Nereis (Fig. 63, p. 97), and they also resemble the lateral eyes of spiders. 

 They are of the simple, upright type, there being : (1) a corneal lens, 

 formed by cuticle and hypoderm cells ; (2) a central vitreous body ; 

 (3) a retina consisting of an outer, clear, bacillary layer, an intermediate 

 pigment zone, and an inner nerve-fibre layer containing the nuclei of the 

 sensory cells. The optic nerve passes directly into the central neuropil 

 of the supra-oesophageal ganglion or brain. 



The development of Peripatus was investigated by Sedgwick, who 

 showed that the eyes are formed at the antero-lateral ends of the two 

 cerebral grooves, which later become closed off from the exterior and 

 form for a time hollow outgrowths (optic stalks) from the brain (Fig. 76) ; 



LAYER OF RODS-- 



PIOMENT L 

 CANCLION CELLS«2s^ 



CORNEAL-LENS & 

 -.-.-VITREOUS 



-OPTIC NERVE 



NERVE-CELLS OF THE 

 5UPRA-0ES0PHAGEA! 

 GANGLION 



Fig. 75. 



Transverse Section through the Head of Peripatus, a Primitive Type of 

 Arthropod, showing the General Arrangement of Nerve Cells in 

 the " Brain " or Supra-cesophageal Ganglia of Invertebrates. (R. J. G.) 



later the cavity disappears and the optic nerve becomes solid. In Agelena 

 (cellar spider) the posterior ends of these cerebral grooves give rise to the 

 central or median pair of ocelli (Kishinouye). 



The development of Peripatus, which was described by Balfour in 

 1 88 1, is extremely interesting from the morphological standpoint, since 

 in many ways it resembles that of vertebrates. The importance of the 

 comparison of invertebrate development with vertebrate development 

 becomes still more obvious when the details of development of other 

 arthropods such as that of certain insects, e.g. Hydrophilus, and of arachnids 

 are taken into consideration. To do this it will be necessary to recall 

 some well-known points of resemblance between the early stages of 

 embryonic development of arthropods and vertebrates in order to 



8 



