the body in locomotion. They are supplemented in 

 the head by other muscles serving to move the grasp- 

 ing hooks. 



An arrow worm's digestive tract is a straight tube 

 from mouth to anus. Often it is the most conspicuous 

 part of the animal simply because the small crea- 

 tures being digested in it have not yet achieved the 

 degree of transparency of the predator surrounding 

 them. 



Arrow worms resemble chordates in having a skin 

 that is several cells thick in some areas and in pos- 

 sessing a tail posterior to the anus. The body cavity, 

 moreover, arises during embryonic development in 

 the manner characteristic of echinoderms and chor- 

 dates but no other phyla. Yet the chaetognaths have 

 no separate circulatory system nor respiratory or 

 excretory mechanisms. Instead, the fluid in the body 

 cavity is propelled by cilia and by movements of the 

 body as a whole, and serves to transport food and 

 wastes from the digestive tract to the body wall, and 

 oxygen absorbed from the sea in the other direction. 



Actually, the body cavity is cut into a head portion, 

 a trunk portion, and a tail portion by transverse par- 

 titions, and is separated incompletely into a right side 

 and a left by a perforated longitudinal sheet of tissue 

 ( mesentery ) holding the digestive tube in place. 



The tail cavity contains a pair of testes, from 

 which the sperm cells escape through ruptures after 

 being coated with mucus to form spermatophores. 

 The trunk cavity, on the other hand, contains a pair 

 of slender ovaries, with ciliated oviducts opening to 

 the outside of the body. Hence an arrow worm is a 

 hermaphrodite, with a male tail and a female trunk. 

 The fertilized eggs are discharged and develop while 

 floating in the water. In many features the embryos 

 resemble those of echinoderms and chordates. 



Probably the planktonic genera Sagitta. Eukroh- 

 nia. and Pierosagitta include only species depending 

 upon self-fertilization. Sagilta is easy to recognize 

 from the two pairs of lateral fins, Eiikruhiiia by the 

 long, slender neck region and single elongated pair 

 of lateral fins, and Pierosagitta from the thick- 

 necked appearance given by a massive collarette 

 extending back to the single pair of small lateral fins. 



Spaclella is a very different arrow worm, asso- 

 ciating with the bottom and clinging to objects there 

 by means of adhesive papillae. In S. cephaloptera 

 these are located on the ventral surface of the tail; 

 other species wear the miniature suckers on finger- 

 like projections situated just in front of the tail fin. 



In all members of Spaclella, the body is more 

 stocky, and the animal spends much of its time hold- 

 ing to a rock or an alga, waiting for food to come 

 within darting distance. Often it seizes victims with- 

 out even letting go of its support. Cross-fertilization 

 is the rule in Spaclella, and the eggs are cemented to 

 the bottom. 



Temperature of surrounding water seems impor- 

 tant to many arrow worms at the time of reproduction. 

 The pelagic kinds that migrate vertically in the trop- 

 ics are exposed to a large range of temperature every 

 day, since at one thousand feet below the surface 

 the water is close to the normal freezing point 

 whereas surface layers may be quite warm. Appar- 

 ently they become dependent upon access to tem- 

 peratures higher than those in the depths, for, if 

 currents carry them into colder water, they fail to 

 reproduce even though they may grow to twice their 

 normal size. 



The arrow worm, S((gi7/rt setosa, occurs in surface wa- 

 ters in incredible numbers at certain seasons. At such 

 times it is an important food for fishes. ( Engliind. 

 D. P. Wilson) 



[145 



