CHAPTER VIII 



The Arrow Worms 



{P/tyliwi CliaetognatJia) 



A, 



.LMOST any bucketful of sea water, whether 

 from the surface close to shore or from the depths, is 

 Hkely to contain a few pencil-slender arrow worms, 

 so transparent as to be overlooked. Even hundreds 

 of them may draw no attention to themselves unless 

 the water in which they swim is poured into a shallow 

 dish with a black bottom. Then the arrow worms 

 show as ghostly, darting creatures from % to 4 inches 

 in length. 



All of the thirty-odd species of arrow worms are 

 free-living. Most of them are pelagic and cosmo- 

 politan. Sometimes they become extremely abun- 

 dant, swimming in great masses that can cloud the 

 water with a grayish tint, particularly at certain times 

 of the day and year. Usually this matches occasions 

 when the sea is locally rich in the favorite foods of 

 arrow worms: microscopic diatoms and other algae, 

 protozoans, copepod crustaceans, and larvae of 

 many other animals, including fish. Toward all of 

 these an arrow worm is a formidable predator, but 

 to jellyfishes, ctenophores, small fish, whale sharks, 

 and whalebone whales, it is merely part of the nour- 

 ishing plankton. 



As an arrow worm rests quietly in the water, its 

 body ordinarily is straight and horizontal. Folded 

 compactly under a thin rounded hood at the anterior 

 end is a pair of sickle-shaped hooks set with movable 

 spines. These serve as jaws when the hood is turned 

 back and the arrow worm darts for about its own 

 length after prey. Between the hooks and surround- 

 ing the slitlike mouth are dozens of short bristles. 



The chaetae from which the phylum Chaetognatha 

 takes its name ("bristle-jaws") are the spines on the 

 prehensile hooks which, when spread and held out 

 stiffly, form the most conspicuous feature of an 

 arrow worm's head. Closer inspection, however, soon 

 leads to discovery of two widely spaced clusters of 

 simple eyes (ocelli), each cluster roofed by a three- 

 part, light-collecting lens. The largest part faces 

 somewhat to the side. A diminutive brain may be visi- 

 ble, connected by very fine nerves to the eye clusters, 

 to the muscles controlling the grasping hooks, and to 

 a narrow organ on the midline believed to apprise 

 the animal of chemical substances in the water — the 

 aquatic equivalent of the senses of taste and smell. 



Fully half of an arrow worm's body is trunk, set 

 off from the head by a slightly narrowed neck and 

 from the tail by another change in body diameter. 

 The sides of trunk and tail bear thin, streamlined fins 

 suggesting the feather vanes on an arrow. From these 

 the principal genus gains its name (Sagitia) and 

 chaetognaths receive the familiar term arrow worms. 

 The tip of the tail also bears a transverse fin. 



Each of the fins is supported firmly by hair-thin 

 rays, but no special muscles permit separate move- 

 ments of these extensions of the body. Instead, they 

 serve in maintaining balance and in making more 

 effective any movements of the body as a whole in the 

 water. 



Except for the fin rays, no structure resembling a 

 skeleton ever develops in an arrow worm. The mus- 

 cles are chiefly longitudinal ones, used in bending 



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