770 



TEXTBOOK OF ZOOLOGY 



them for food. The zone extends from shorelme to a depth of from 

 50 to 75 feet. (2) Intermediate (sublittoral) zone includes relatively 

 shallow water of from 50 to 500 feet and supports hydroids, sponges, 

 some corals, sea cucumbers, sea urchins, and starfish. There is a grad- 

 ual increase in stability of conditions through this zone. (3) Bathyal 

 (abyssal) zone is in the depths of the ocean ranging from 5,000 to 



1. 



3. 



9. 



10. 



11. 



12. 



Fig. 405. — A group of typical mollusks occurring along- the Texas coast of the 

 Gulf of Mexico. (From specimens belonging to Dr. Elmer P. Cheatum). 1, Solen 

 viridis. 2, Dosina discus. S, Seniicassis gibba. !i, Oliva sayana. 5, Donax roenieri. 

 6, Venus campechiensis texana. 7, Crepiduln fornicata. 8, Polinices duplicnta. 

 9, Thais haeniostoma floridana. 10, Architectonica granulata. 11, Littorina iiTO- 

 rata. is. Modiolus tulipus. 



25,000 feet. The pressure increases fifteen pounds per square inch with 

 each thirty feet of increased depth. The conditions become even more 

 constant in this zone. Below 800 feet there is no light and enormous 

 pressure. In the great depths the temperature is nearly constant 

 and is about 2° C, or just above freezing. Most of the animals 

 found in this zone are relatively small and somewhat modified 



