PINEAL AND PITUITARY BODIES OF THE BRAIN. 21 
(fig. 2) the vertebrate paired eye, (fig. 8) the vertebrate 
pineal eye, and (fig. 4) a well developed invertebrate eye, 
and show that such important differences exist between 
them all that the pineal eye cannot be considered as 
resembling either the paired vertebrate eyes or the 
ordinary higher invertebrate eyes, but must be regarded 
as a type distinct from both. 
It may be pointed out, as an instance of the important 
differences between the three eyes, that the large central 
cavity between the lens and the retina (see Pl. L,, figs. 
2, 3, 4) is of an entirely different nature and origin in 
each. In the pineal eye (fig. 3, C(n).) it is a part of the 
original cavity of the neural canal, and is surrounded by 
epiblast. In the paired vertebrate eye (fig. 2, C(v).) the 
central cavity (occupied by the vitreous humour) is formed 
in the mesoblast, and has nothing to do with the neural 
canal; while in the invertebrate eye (fig.4, C(e).) the cavity 
is outside the epiblast and is bounded in front By the 
thickened cuticle. 
The second point is, that Spencer comes to the con- 
clusion (see l.c., p. 232) that ‘‘ The epiphysis of higher 
Chordata is the homologue of the larval Tunicate eye.” 
This homology I am greatly inclined to doubt, and 
although it may not be possible at present to disprove it, 
still I think that the evidence in favour of it is insufficient, 
and that therefore it cannot be accepted. 
It may be pointed out (1) that the lens in the eye of the 
larval Tunicate is not cellular, and (2) that if the Tunicate 
eye was pushed out to form a vesicle the retina would be 
found in the position occupied by the lens in the pineal 
eye (see Pl. I., fig. 5). Spencer recognized both these 
difficulties, but apparently did not consider them of 
sufficient importance to destroy the homology. 
Iam at present inclined to regard the sense-organs of 
