8 PAL/EONTOLOGV 



foramen in the ventral valve. It feeds entirely upon 

 the microscopic organisms, both animals and plants (but 

 probably in the main the minute plants known as 

 diatoms) which drift about helpless in the currents in 

 the water. So enormously abundant are such organisms 

 in the ocean, and so quickly do they grow and reproduce 

 themselves, that they form the food, at first, second, or 

 third hand, of nearly all the animals in the sea. It has 

 well been said that " all fish is diatom " in the sense that 

 " all flesh is grass," for there is nothing in the ocean 

 comparable to the vegetation of the land. The larger 

 sea-weeds do not afford food to many of the marine 

 animals. 



An animal feeding upon such helpless microscopic 

 prey can do without many of the organs that are essential 

 to the familiar animals of the dry land eyes and ears to 

 see their prey, arms and jaws to seize them, swimming or 

 running organs to pursue them, are all needless. Shut 

 up within its shell a brachiopod is comparatively safe 

 from enemies ; fixed by its pedicle, only very violent 

 storms can throw it up to die on the shore. All it needs 

 in external organs are (i) some means of creating a 

 constant current into and out of its shell-cavity, to bring 

 in food and oxygen and carry out waste products ; (2) a 

 shell so arranged that it can be closed tightly when 

 necessary for safety, and opened at will to allow ingress 

 and egress of the water-currents. These necessities 

 control and determine the structure both of the soft parts 

 of the animal's body and of the shell which it secretes. 

 The actual " body " of the terebratuloid, containing 



