194 ONYCHOPHORA. 



the jaws of Peripatus are to be homologised, not with the mandibles of the 

 Insecta, hut rather with the second antennae of the Crustacea. The question 

 which naturally arises as to whether the corresponding segment has been lost 

 in the Insecta, or, in other words, as to the relation to it of the mandibular 

 segment, can hardly, in the present state of our knowledge, be profitably 

 discussed. 



The close connection brought about between the maxillary segment and the 

 cephalic segment increases the probability of the view expressed above, that 

 the autennal segment also (now known as the cephalic segment) may have 

 been united with a cephalic section formerly present, and now to a great extent 

 degenerated. "We were led to this assumption by the presence of the two 

 prominences in front of the antennal rudiments (Fig. 94, x), and by the close 

 agreement in manner of formation between the antennae and the feet. It is, 

 indeed, difficult to reconcile with this view the statement that the antennal 

 nerve forms in a manner essentially different from the peripheral nerves, but 

 this point has as yet received too little attention to be considered as of decisive 

 importance. 



The Eyes. 



The rudiments of the eyes have already appeared before the 

 separation of the nervous system from the ventral organs. On 

 the dorsal boundary of the ectodermal thickening in the cephalic 

 segment, a small pit is formed on each side, behind and somewhat 

 ventrally to the rudiments of the antennae ; the floor of this pit 

 is at first connected with the ectodermal thickening, but soon 

 becomes detached from it. The pit closes to form a vesicle, which 

 becomes constricted off from the ectoderm. Outwardly, i.e., towards 

 the epidermis, this vesicle is unilaminar, but on the inner side it 

 is multilaminar. Pigment appears on the inner boundaries of its 

 cells, and in its cavity the lens is secreted. The cells of the inner 

 and lateral walls yield the rods of the retina. A differentiation into 

 cell- and fibre-substance has already taken place in the thickened 

 inner wall of the optic vesicle, and a connection which occurs 

 between this part and a process sent out by the brain gives rise 

 to the optic nerve, which is thus a secondary formation (v. Kennel). 



Sedgwick's account of the origin of the eyes in Peripatus is somewhat 

 different. According to him, the region in which they originate still belongs 

 to the brain, and they do not lose their connection with the latter, the inner 

 wall of the optic vesicles remaining united with the cell-mass of the brain. 

 The optic nerve arises at this point later by simple constriction. The eyes 

 thus originate chiefly from the brain, and are covered merely by the ectoderm 

 of the surface ; they are "cerebral eyes," according to Sedgwick, in opposition 

 to v. Kexxel, who believes, as stated above, that they arise independently 

 of the brain. 



It is possible that the observations made on the origin of the eyes in Peripatus 

 can be harmonised with those of the development of the eyes in the Annelida. 

 The eyes of Peripatus agree closely witli more highly organised Annelidan eyes, 



