62 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



that lie ventral to the optic lobes, showing that the antennae have 

 migrated forward beneath the eyes, and not above them as in the 

 Onychophora. Moreover, in the arthropod brain the antennal glomer- 

 uli are not immediately connected with the corpora pedunculata. The 

 onychophoran brain in its modern form, therefore, could not have 

 given rise to a brain of arthropod structure, and we can assume only 

 that the two types of cerebral structure have taken their origins 

 separately from some common progenitor not far removed from a 

 generalized annelid. Even the inclusion of the nerve centers of the 

 first postoral somite in the onychophoran brain cannot be taken as 

 evidence that the Onychophora are ancestral to the Arthropoda, for 

 in some of the lower members of the second group the first postoral 

 (tritocerebral) ganglia are not united with the brain. 



THE EYES 



The eyes of the Onychophora resemble the eyes of annelids in 

 structure and development. An eye of the annelid-onychophoran 

 type is formed from an invagination of the body wall (fig. 28 C), 

 which becomes closed by an approximation or union of its lips (D, E), 

 thus producing an inner optic vesicle (OpV) beneath an outer layer 

 of epidermis and corneal cuticula (Cor). The cavity of the vesicle 

 is occupied by a crystalline lens (Ln), probably of a cuticular nature, 

 and its inner wall becomes the retina (Ret). In the onychophoran 

 eye (A), as described by Dakin (1921), the lens is strongly convex 

 outwardly and rests on the thick retina (Ret). Each retinal cell (B) 

 is differentiated into a distal cylindrical rod (c) and a basal pigmented 

 part (d), which contains the nucleus (Nu), and is prolonged proxi- 

 mally as a nerve fiber (;//) that enters the optic lobe of the brain. 

 The rods appear to have peripheral striations (e), but, as shown in 

 cross-section (F), they do not form structures between them corre- 

 sponding with the rhabdoms of arthropod eyes. 



LATER HISTORY OF THE MESODERM AND THE COELOMIC SACS 



The mesoderm bands of the Onychophora in their forward growth 

 (fig. 22 B, C) continue into the head, where they form a pair of 

 distinct coelomic sacs in the antennal region diverging anteriorly from 

 the mouth (D). The cephalic coelomic sacs are described by Sedg- 

 wick (1887) m Peripatopsis capensis, by Kennel (1888) in Peripatus 

 edwardsi, and by Evans (1902) in Eoperipatiis tueldoni. The sacs 

 are at first of large size (fig. 2"/ B) ; posteriorly their splanchnic walls 

 embrace the stomodaeum (Sfom) and give rise to a part of the 



