MARINE ZOOLOGY 



771 



structurally as well as physiologically in order to adjust to the con- 

 ditions. Approximately 150 species are known to exist at 15,000 

 feet or deeper. There is a tendency toward the development of 

 phosphorescent organs and telescopic eyes among animals inhabiting 

 the bathyal regions. 



Fig. 406. — Common bivalve and univalve mollusks that occur along the Texas 

 coast of the Gulf of Mexico. (From specimens belonging to Dr. Elmer P. Chea- 

 tum.) 1, Dinocnrdiuni robustum. 3, Busucon perversus. 3, Barnea costata. 

 i, Ostraea virginica. 5, Plagioctenium irradiens. 6, Atrina seminuda. 



The methods of studying the conditions of the ocean are largely 

 by survey methods, use of light disks, pressure gauges, electric 

 thermocouple temperature recorders, hydrogen ion analysis ap- 

 paratus, oxygen tension analyses, and chemical analysis for salinity. 

 Samples of sea water can be obtained from various depths by use 

 of a sampling tube or cylinder which is lowered while open so that 

 water passes right through it. When the sample is desired, a lead 

 messenger is sent down the cable to trip the trigger and allow the 

 cylinder to snap shut at both ends. The cylinder with the sample 



