LUMINOSITY OF MARINE ANIMALS AND INSECTS. 329 
396 . But the hfoctilucse and Acalephae are by no means the 
only luminous animals which tenant the deep. Many Zoo¬ 
phytes appear to have this property in an inferior degree, and 
also some of the Echinodermata. Of the lowest class of 
Mollusks, the Tunicata, a very large proportion are luminous, 
especially those which float freely through the ocean, and 
which abound in the Mediterranean and tropical seas ; the 
brilliancy of some of these can scarcely be surpassed. Among 
some of the Conchifera, also, the phenomenon has been ob¬ 
served ; as well as in certain marine Annelida. Other marine 
animals of higher classes are possessed of similar properties; 
thus, many Crustacea , especially the minuter species, are 
known to emit light in brilliant jets ; and the same may be 
said of a few Fishes: but it is probable that the luminosity 
attributed to many of the latter is due to the disturbance they 
make in the surrounding water, which excites its phospho¬ 
rescence in the manner just explained. In all these, the 
general phenomena are analogous,—the luminous matter ap¬ 
pearing to be a secretion from the surface of the animals, and 
to undergo a sort of slow combustion by combination with 
oxygen. Wherever it is presented by these animals, it is 
always most brilliant upon the surfaces concerned in respira¬ 
tion. The light continues for some days after death ; but 
ceases at the commencement of putrefaction. 1 
397. In the class of Insects, there are several species which 
have considerable luminous power; and in these the emission 
of light is for the most part confined to a small part of the 
surface of the body, from which it issues with great brilliancy. 
The luminous Insects are most numerous among the Beetle 
tribe, and are nearly restricted to two families, the Elateridce, 
and the Lampyridce . The former contains about 30 luminous 
species, which are known as fire-flies ; these are all natives of 
the warmer parts of the JSTew World. Their light proceeds 
from two minute but brilliant points, which are situated one 
on each side of the front of the thorax; and from another 
1 There are certain cases, however, in which the production of Light, 
like that of Electricity (§ 423), appears to be a peculiar manifestation 
of Nervous power. There is strong reason to believe that Nerve-force 
may be directly metamorphosed (as it were) into these or other forms of 
physical and vital force, according to the principle of “ Correlation ** 
now generally admitted as regards the Physical Forces. 
