Chap. 27 AN AQUATIC MISCELLANY 551 



The bristles and hooks that surround the head of an arrow worm are in- 

 stantly recognizable as the tools with which as a carnivore it seizes its prey 

 (Fig. 27.16). The body cavity, a true coelom, is divided into compartments, 

 all lined with peritoneum and filled with peritoneal fluid. Arrow worms are 

 pioneers in the development of a coelom, and this possibly places them among 

 the transitional forms from which the ancestors of the vertebrates finally 

 emerged. There are no special respiratory, circulatory, or excretory organs 

 but diffusion through the whole body carries on their work. 



Arrow worms are hermaphroditic. The ovaries are in the coelomic cavities 

 of the trunk, and the testes in coelomic cavities in the tail. At hatching, the 

 young resemble the adult. 



Phylum Phoronidea 



The special features of phoronids are the food-catching organ, the body 

 fluids, the coelom and the larva (Fig. 27.17). The food collector is a lo- 

 phophore on a larger scale but similar to that of rotifers in its structure and 

 importance to the welfare of the animal. There are two body fluids, a color- 

 less one in the body cavity, and red blood circulating in blood vessels. Both 

 fluids are very different from those of higher animals, the lymph and blood 

 which they suggest. 



