550 EVOLUTION OF ANIMALS Part V 



for a brief time on microscopic organisms — crustaceans and larval fishes, and 

 then return to the dark deep water. They not only furnish food to animals a 



.^^^^^^^Snn 



Fig. 27.16. Arrow worms (Sagitta hexaptera) swarm in open seas suddenly 

 visiting the surface at certain times of the year and during morning and evening 

 twilights. This species (length 3 inches), among the largest of the arrow worms, is 

 abundant off Martha's Vineyard, Massachusetts, and occurs throughout the world. 

 (Courtesy, Miner: Fieldbook of Seashore Life. New York, G. P. Putnam's Sons, 

 1950.) 



little larger than themselves, but a few billion of them make a tasty catch for 

 the whale-bone whale whose food sifter is as efficient for gallons as that of a 

 rotifer for droplets. 



Structures and Functions. The phylum name refers to the bristly mouths 

 and that of the principal genus, Sagitta, to their habits of darting like arrows. 



Anus 



Mouth 



Cfrculotor 

 system 



Esophog 



ectum 



Ovary 



entory 

 not 



Stomach 



Fig. 27.17. Phoronis, a tube-dweller in 

 the mucky sand between the tide lines. 

 Diagram of its structure; the crown of 

 sticky tentacles is its all-important means 

 of getting a living. It is chiefly interesting 

 as a link suggesting relationships of var- 

 ious phyla of invertebrates and even a 

 remote one with the chordates because of 

 a notochord-like structure present in 

 them. (Courtesy, Hunter and Hunter: 

 College Zoology. Philadelphia, W. B. 

 Saunders Co., 1949.) 



Testis 



With their crystal transparency and cutting speed they are more like glass- 

 arrows than arrow worms. Dozens of them may swim about unseen in a glass 

 of water. 



