340 . JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



solely from a comparative study of the eye tends. In order to deter- 

 mine the actual relationships it would be necessary to determine the 

 relationships indicated by the comparative study of each of the sys- 

 tems of organs and then to compare results. The study of the eye 

 does, however, offer especially favorable conditions for judging of 

 relationships between species, and the evidence from this source is 

 worthy of emphasis. 



What is the fundamental plan of the eye of salpa? 



The eye is formed from the central nervous system and not directly 

 from the ectoderm, conforming in this respect to the vertebrate rather 

 than the invertebrate type. It is a simple eye, readily comparable in 

 structure to the pineal or lateral eye of vertebrates. It is composed of 

 a series of optic units, each of which consists of a rod cell and one or 

 more pigment cells. That end of the rod cell which receives the inner- 

 vating fiber is thin-walled and contains the nucleus. The other end of 

 the rod cell which is near the pigment cells is thick-walled, resembling 

 the thick-walled ends of the vertebrate rod and cone cells. The following 

 diagram will show the relation between the structure of the salpa eye, 

 C, and that of the vertebrate lateral or pineal eye, D. 



The resemblances are seen to be considerable. The rod cells, the 

 essential element in each, are alike, and the innervation is the same. 

 The optic unit of the vertebrate eye is more complex than that of salpa, 

 having certain ganglion cells interpolated between the rod cells and the 

 brain ; but the fundamental character of the two is the same. The eye 

 in the two groups is formed by a similar modification of the cells of the 

 central nervous system, this modification having gone further in the 

 vertebrate than in salpa. Although the eye of salpa so closely resembles 

 the vertebrate eye in structure, it cannot be regarded as homologous with the 

 latter. It has often been homologized with the ascidian larval eye and 

 the vertebrate pineal eye, but such an homology cannot be sustained, as 

 we will see after we have studied the relation of the ascidian nervous 

 system to that of salpa. We will see that the eye of salpa is derived 

 from a portion of the nervous system not represented in the ascidian 

 tadpole or in vertebrates. This will be taken up in connection with the 

 study of the subneural gland. 



We have in the chordata a series of variations in the structure of 

 the optic organs. Salpa eye falls into this series, although it is not 

 homologous to the optic organs of any other chordate. In amphioxus 

 we find merely a slight pigmentation of the inner ends of certain cells in 



