Chap. 27 



AN AQUATIC MISCELLANY 



549 



Modern (A) and Fossil (B) 

 Brochiopods 



Fig. 27.15. A, Lingula, so like its ancestors that it is called a "living fossil," 

 still so abundant on the borders of the Indian Ocean that it is used for food. It 

 lives in vertical burrows in the sand attached to the bottom by a stalk. B, a fossil 

 brachiopod shell that displays marked likeness to living brachiopods. (After Pauli: 

 The World of Life. Boston, Houghton Mifflin Co., 1949.) 



enormous lips (lophophore), that surround the small mouth and bear rows of 

 ciliated tentacles. When not in action, the lips or "arms" are coiled up on each 

 side of the mouth. Their many tentacles have ciliated grooves through which 

 food and water are drawn toward the mouth. The cavity within the shell is 

 divided into a front chamber containing the lophophore and the lobes of the 

 mantle, and a posterior one containing the coelom, branches of which extend 

 into the mantle. It also contains the pairs of muscles by which the shell is 

 opened or closed and turned on its stalk, also those that work the stalk of such 

 burrowers as Lingula. The digestive canal usually lacks an anal opening. Any 

 waste which remains after digestion must be exceedingly fine, probably dis- 

 solved and excreted by the two relatively large nephridia. The sexes are sepa- 

 rate. Fertilization of the egg occurs outside the body. The free-swimming larva 

 is ciliated and has a general resemblance to the trochophore larvae of annelid 

 worms, rotifers, and moUusks (Fig. 27.1). 



Phylum Chaetognatha — Arrow Worms 



Their Vertical Migrations. In the morning and evening twilights, vast num- 

 bers of arrow worms join the plankton population of the sea. There they feed 



