30 
IOWA ACADEMY OP SCIENCES. 
largely of water, which is nearly incompressible, while many 
invertebrates possess abundant skeletal tissues of limestone 
usually permeated by profusely branching canals containing 
watery fluid (ecbinoderms, corals, etc ), or consisting of small 
particles or spicules, embedded in a watery coenasarc (alcyon- 
aria, sponges), or with an external chitinous investment 
(Crustacea). 
When fishes are brought up from great depths their tissues 
almost fall apart, on account of the release of pressure; the 
swim-bladder projects from the mouth and the eyes are greatly 
protruded. 
Second. — Deep-sea animals are subjected to a comparatively 
uniform low temperature. This temperature is between 38 
degrees and the freezing point at all places below 150 fathoms. 
As we near the poles this low temperature approaches, and 
finally reaches, the surface. 
Third . — Absence of wave novements. In many places, how- 
ever, there is a steady mass-movement of the water in the 
shape of currents. 
Fourth. — Practical absence of sunlight. By this I mean that 
the light penetrating to a depth below 100 fathoms can not be 
regarded as sufficient to enable such eyes as ours, and probably 
all eyes, to distinguish between colors.^ 
Fifth . — The presence in many places of animals giving forth 
phosphorescent light. This being an important phenomenon 
for our purpose, I have gathered together considerable evi- 
dence to show the extent to which this light-emitting power 
prevails among abyssal forms. It seems that phosphorescent 
light is found among the following groups of deep-sea 
animals: Fishes, diXong the lateral organs or on the head. 
Salim; the Blake expedition secured specimens which were 
several yards in length and highly phosphorescent. Many 
crustaceans, cephalopods. Among the Challenger material were 
specimens having very efficient phosphorescent organs on the 
lower surface, and not only was the light emitted, but lenses 
were found for concentrating the light as does a bull’s eye 
lantern, f Ophiurians, Pennatulidce, which are described as 
* Professor Verrill, however, maintains that a pale green lig’ht penetrates even to 
great depths. (See report of Commissioner of Fish and Fisheries, 1882, pp. 1054-1056.) 
This point will be discussed later. 
t These remarkable structures were described before the zoological section of the 
A. A. A. S. at the Detroit meeting by Prof. William E. Hoyle, in a paper that was not 
published. 
