MORPHOLOGICAL STUDIES 
55 
Morphological Studies. 
No. I.— The Parietal Eye of the Cyclostome 
Fishes. 
By 
J. Heard. Ph.D., B.Sc. 
With Plates VI and VII. 
Introduction. 
The discovery of the parietal eye by de Graaf (No. 7), and 
the beautiful account of its structure in a great many genera of 
Sauria by Spencer (No. 14) are of so very recent occurrence, 
and excited so much interest among zoologists, that I can 
refrain from an historical account of their work and of previous 
researches on the pineal body, with which this sense organ is 
identical. Spencer has given a full statement of all that was 
known of this organ in Lizards, and 1 shall therefore only pre- 
face the following account of its structure in the Cyclostomata 
by a brief notice of Ahlborn’s work on the subject. In his 
paper on the brain of Ammoccetes Wiedersheim (No. 16) 
had described the presence of a greyish-white pigment in the 
pineal body, and this discovery was confirmed by Alilhorn 
(No. 1, p. 233). Ahlborn has, in addition, given some account 
of the minute structure of the organ in both Ammocoetes and 
Petromyzon, and, indeed, as the result of his studies he pub- 
lished in a separate essay (No. 2 ) some views on the nature of 
the pineal body — independently of, but identical with, those 
enumerated by Rabl.-Riickhard (No. 15) some time before. 
