RELATION OF MEDIAN TO LATERAL EYES 359 



one of the most important pieces of evidence in favour of the vertebrate 

 stock having arisen from an arthropod ancestor. The controversy was 

 chiefly centred on the general resemblance in the form and structure of 

 certain palaeozoic Merostomata and their living representatives, e.g. Apus, 

 Limulus, and Scorpio, to the Ostracodermata and their living repre- 

 sentatives, namely, the cyclostomes. The Merostomata which were 

 specially cited included Bunodes, Eurypterus, and Pterygotus, which along 

 with the " trilobite larva " of Limulus, were compared with Cephalaspis 

 and Auchenaspis (Thyestes). A closer study by recent workers (Stensio, 

 Kaier) involving the comparison of the central and peripheral nervous 

 systems, the sense-organs, and other parts of living cyclostomes (see 

 Fig. 22, Chap. 3, p. 28) with reconstructed casts of the cranial cavities 

 of certain Palaeozoic fishes (Anaspida, Cephalaspidomorpha, Fig. 245), 

 has definitely shown that the claims made by earlier authors of a close 

 relationship between these ancient fishes and the cyclostomes were well 

 founded. Moreover, a more exact knowledge of the mode of development 

 of the median and lateral eyes of invertebrates has led to a better under- 

 standing of the differences which exist between the fully developed eyes 

 of different types of the adult animal ; also the mode of development of 

 the more complicated types from the simpler, and the way in which an 

 eye commencing as an epithelial pit and primarily purely dermal in 

 origin may by a process of inrolling of the skin in the region of the neural 

 crest be carried towards the median plane and ultimately included in the 

 membranous roof of the fore-brain vesicle (Fig. 246), where later being 

 cut off from the skin in the process of closure of the neural tube, it finally 

 appears as a tubular evagination of the brain. The distal end of this 

 tube, which corresponds to the bottom of the primary dermal pit, is usually 

 dilated and forms a vesicle in the walls of which there are a variable 

 number of retinal placodes (two, three, or four), which are commonly 

 grouped or fused into a single median eye. These arise as one or two 

 pairs, the tri-placodal type being formed by the complete fusion of one 

 pair of placodes and the incomplete fusion of the other pair, which usually 

 lie dorsal to the single placode (Figs. 248, 249). In the process of inrolling 

 of the ocellar pit either the superficial or the deep limb of the fold may be 

 developed as the sensitive or retinal layer (Fig. 246, A and B). If the 

 deeper limb of the fold is modified to form the retina the superficial 

 or pre-retinal layer may atrophy or it may be utilized in the forma- 

 tion of a lens. Whereas if the more superficial limb of the fold is trans- 

 formed into the sensitive layer, the deeper or post-retinal stratum may 

 give rise to a pigment layer or a reflecting membrane (tapetum). A 

 more common arrangement is for only the deeper part of a symmetrically 

 formed optic pit to be modified as a sensitive or retinal area, the cells at 



