234 FREDERICK TILNEY AND LUTHER F. WARREN 



Mathias Duval 98 in 1888 brought to a conclusion a notable 

 series of lectures with the statement that the history of the 

 pineal gland has played an important role in the study of homolo- 

 gies in the structure of the vertebrates and invertebrates. 

 He further states that the situation of the pineal body in rela- 

 tion to the nervous system of vertebrates and in comparison 

 with the oesophageal ring in invertebrates gives the structure 

 a new significance. From this it might be possible to determine 

 one of the clews which should reveal how the vertebrates resulted 

 from the successive transformation of the invertebrates. 



Several years prior to this observation, Ahlborn 2 suggested 

 that the parietal organ was comparable to the unpaired eye of 

 amphioxus and tunicata, while Rabl-Ruckhard 322 was of the 

 opinion that an homology existed between the pinal organ and 

 the parietal eye of arthropoda. Baudouin 15 expressed the view 

 that of the proto-vertebrates, larval ascidians possess an un- 

 paired eye which, however, disappears in the adult. This organ 

 is situated immediately beneath the epidermis and consists of 

 a retina, a lens, and a pigment layer. It is derived from the 

 cerebral vesicle and supposed to be the vestige of a transitory 

 eye which previously existed in adult ascidians. Indeed, in 

 pyrosomes this unpaired eye is well developed in the adult, 

 possessing a retina, lens, and optic nerve.. There are no lateral 

 eyes in these invertebrates, and hence the unpaired eye must 

 functionate as a visual organ. In tunicates there exists both 

 the paired and unpaired eyes. In amphioxus there is a pig- 

 mentary patch placed above a dilation of the brain, but one 

 is not justified in considering this the homologue of the unpaired 

 eye in tunicates. 



Peytoureau 3083 held the opinion that the pineal eye exists in 

 vertebrates in a degenerated state only. In extant forms of 

 the tunicates it still exists as a functional organ, occupying in 

 these animals almost exactly the same position and having the 

 same disposition as in lizards and amphibia. In tunicates there 

 is an unpaired eye and two paired eyes which he believes func- 

 tionate simultaneously, the unpaired eye being comparable to 

 the parietal eye of the lizard and amphibia not only because of 



