30 IOWA ACADEMY OF 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 (echinoderms, 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. 



Secojtd. — 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. 



FourfJt. — 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: Fi.shes, along the lateral organs or on the head. 

 Salpa: the Blake expedition secured specimens which were 

 several yards in length and highly phosphorescent. Many 

 (•niHtaveauH, ceiihahiiods. 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, t OphiHricnt.s, Pennatidida', which are described as 



♦Professor Verrill, however, maintains that a pale green ligrht penetrates even to 

 great depths. (See report of Commissioner of Fish and Fisheries, 18&2, pp. 1054-105(5.) 

 This point will be discussed later. 



• These remarlcable structures were described before the ziological section of the 

 A. A. A. S. at the Detroit meeting by Prof. William E. Hoyle, in a paper that was not 

 published. 



