582 COMPARATIVE ANATOMY 



Urochordates have pigment spots and an unpaired eye, neither of 

 which can be compared with those of vertebrates. Such forms as salpa 

 have a median vesiculated eye derived from the brain and usually pig- 

 mented. Only in their origin from the brain do such eyes resemble those 

 of vertebrates. 



Among cephalochordates the so-called eye of amphioxus is a pigment 

 spot located at the anterior end of the nerve cord. There is obviously 

 very little resemblance between such a structure and the eye of a verte- 

 brate. The reasons for calling it an eye are that pigment is associated 

 with eyes, and that this special pigment spot is associated with the anterior 

 expansion of the nerve cord, which is generally homologized with the fore- 

 brain of vertebrates. On the basis of such slight resemblance, it is 

 scarcely possible to derive the paired vertebrate eyes from this unpaired 

 pigment spot. 



In the floor of the nerve cord of amphioxus, throughout its length, are 

 photoreceptors partially enclosed by pigment capsules. Since removal 

 of the pigment spot from the so-called brain of amphioxus does not affect 

 its response to light, it is assumed that the true light-recipient organs of 

 this animal are these photoreceptors of the nerve-cord. 



The study of invertebrates reveals that their eyes have all the histolog- 

 ical elements of the paired eyes of vertebrates, but never in the same 

 combinations. Some invertebrates have photoreceptors in the form of 

 rods and cones associated with pigment spots and with the brain. But 

 vertebrates alone have eyes with an inverted retina formed as an out- 

 growth of the brain wall, and surrounded by a mesenchymatous capsule. 

 Comparative anatomy throws little, if any, light upon the past history 

 and origin of paired eyes, since the eyes of the cyclostomes are in all essen- 

 tials like those of the highest vertebrates. Eyes, therefore, appear to 

 spring into existence full-formed, and we are compelled to draw phylo- 

 genetic conclusions from the facts of ontogenesis. 



These facts appear to justify the conclusion of Ray Lankester (1880) 

 that paired eyes of vertebrates are paired pigmented depressions in the 

 anterior part of the neural plate. It has been asserted that the parietal 

 eye has a similar paired origin, from pits anterior and lateral to those which 

 form the paired eyes; and the conclusion has been drawn that, when the 

 neural plate was converted into a neural tube, the unpaired eye was 

 formed by the fusion of the lateral paired pits, which subsequently grew 

 out as a stalked vesicle. Thus the parietal eye looks upwards, while the 

 paired eyes, bulging laterally from the brain wall are receptors of light 

 from the sides and below. In most vertebrates, the median eye degen- 

 erates ; but the paired eyes enlarge and become the definitive organs of vision . 



It is fairly easy to imagine the conditions which led to the lateral 

 outgrowth of the paired eyes. Among the factors involved was presum- 



