G05 
is <iuitc cijibryonic, and the lens altogether absent. A parallel case 
is i)resented among ourselves by the common Mole, the eyes of 
A\hich show similarly an arrested embryonic chai’acter. 
iSeveral species of blind fish are known from the caves in 
America and Cuba, all of which are white in colour. Some of 
them, however, possess eyes which are imbedded deeply in the 
tissues of the head. Many blind insects are met with ; in 
some species the eyes arc rudimentary, in others entirely wanting, 
even in the larval condition. On the other hand, there are found 
some spiders which possess eyes, although they cannot apparently 
1)0 of any service to them. Perhaps such cases may be explained 
by the supposition that these forms are compamtively recent immi- 
grants, or that some animals may bo subject to less modification 
than others, oven when exposed to the .same conditions. A 
consideration, however, of the whole subject, shows that, in the case 
of animals inhabiting caves, there is a tendency towards the 
reduction, or atrophy, of their visual organs. 
The same result is arrived at by the study of the deep-sea fauna, 
altliough wo find a somewhat anomalous mixture of blind forms 
with others possessing moderately developed or sometimes enormous 
eyes. This is probably to bo accounted for by the fact, that a large 
number of animals inhabiting great depths are phosphorescent. 
Many of the deep-sea Cadenterates are luminous, such as the Sea- 
pens (Pennatulidic) and the Gorgonian Corals. Pennatula is 
exceedingly brilliant. Umbollularia, which occurs at depths as 
great as two thousand four hundred fathoms, attains a length 
of three-and-a-half feet, and gives out a very strong light. Many 
deep-sea, and some pelagic fish are phosphorescent. In the former, 
the whole system ot the lateral lino becomes enormously developed, 
with great cavities in different parts of the body, especially in the 
head. These cavities are filled with mucus, which also covers most 
of the body; and in perfectly fresh specimens the mucus is usually 
luminous. Many fish also possess, imbedded in the different parts 
of their skin, particularly in the ventral region, round shining 
bodies, sometimes simple and glandular, without ducts, but 
connected with the spinal nerves ; at other times in the form 
ot chambei’s, filled with fluid, and coated behind by a membrane, 
which in some cases exhibits a structure not unlike that of a 
retina, with a well-developed bi-convex lens in front. These latter 
