156 ORGANIZATION OF ANIMALS LIVING IN DEEP SEAS. 



nites and Nautilites. It is probable that the animals that had straight 

 chambered shells possessed greater facility of rising to the surface 

 than the spiral ones, and accordingly we find them chiefly in the old- 

 est and lowest formations. The animals of this class having heads 

 and various senses seem to rank high in the scale of sentient organ- 

 ic beings ; but they are not numerous, till we rise in the secondary 

 strata, above the coal formation. 



Very few spiral unchambered shells occur in the transition rocks*; 

 for these animals crawl on their bellies like the snail, and do not 

 seem fitted to live in deep water, unless, like the Helix Janthina, 

 which nearly resembles the snail and lives in the Southern Ocean, 

 they had little appendages like bladders, which enabled them to rise 

 to the surface. Univalve unchambered spiral shells, become nume- 

 rous in the upper strata, probably from the circumstance that these 

 strata were deposited under shallower seas. 



With respect to that class of the testaceous Mollusca which did 

 not enjoy the privilege of having heads and eyes, their motives for 

 travelling, whether for pleasure or necessity, must have been few in- 

 deed ; and they may be supposed to enjoy life as well in the deepest 

 recesses of the ocean, as nearer its surface. The tenants of bivalve 

 shells, called by Cuvier, Acephales\, have, however, a power of lo- 

 comotion which* they effect, some by thrusting out a membrane called 

 a foot, and with it thej^ also attach themselves to rocks or other bo- 

 dies, by a number of filaments called the Byssus, which they can 

 remove at pleasure : others have two tubes, with which they force out 

 water with considerable violence, and impel themselves in an oppo- 

 site direction; and others again, by a strong muscular action in open- 

 ing and shutting their shells, can jump twelve inches at one leap. 



All these modes of motion, however, though sufficient for the 

 wants of the animal, are very limited in their operation, and are 

 equally adapted for animals in deep or shallow seas, in rivers or 

 lakes: accordingly, we find numerous testaceous Mollusca of this 

 class, not only in the transition, the secondary, and the tertiary strata 

 but at various depths in our present seas and lakes. 



* All unchambered spirals shells were occupied by animals which had an or- 

 ^an of motion placed under the bodyj as in snails: they had heads, and are called 

 i)y Cuvier, Gaderopodes. 



Acephaies — hailing no heads. 



