THE INVERTEBRATE LEGIONS 



105 



the bodyless head grows a new body— result: two worms. If the head is cut 

 longitudinally in two the result is the regeneration of two heads. 



Almost all free-living flatworms have two eyes on the head which are turned in, 

 making the animal appear cross-eyed. These eyes do not form an image, but 

 inform the worm of the presence of light or dark. In general, flatworms avoid 

 light. They feed on organic matter of all sorts, which they locate by means of 

 chemical reception. Flatworms either glide on their bellies by means of cilia or 

 swim by body undulations. 



The acoel flatworms have no digestive system and range up to a half-inch 

 in length. The red Polychoerus is found in the tidal zones of both coasts. 

 Convohita is a green acoel that derives its color from zoochlorellae in its body. 

 This animal may feed when young, but when it becomes adult, it lives entirely 

 by digesting the zoochlorellae, which results in the death of the worm, an 

 example of poorly balanced mutualism (Buchsbaum, 1948). 



Rhabdocoel flatworms have a straight intestine and are very common on 

 muddy and sandy shores or seaweeds. The triclads have three-branched intestines, 

 are of dark color, and are most common on stony bottoms of northern seas. Both 

 rhabdocoels and triclads rarely reach a half-inch in length. Bdelloura is a triclad 

 common on the gills of the king crab, Lhnidus. 



The polvclads have many-branched intestines and reach a length of a half-foot 

 or more. The tropical ones are frequently very beautiful, matching their sur- 

 roundings in color. Some polyclads are pelagic. They swim by means of wavelike 

 motions of the sides of the body. 



All of the flatworm groups mentioned show tendencies toward commensalism, 

 suggesting that the parasitism so common in other flatworm groups, such as 

 flukes and tapeworms, evolved first from commensalism. The end product of 

 parasitism is often mutualism which, however, is not present in flukes and 

 tapeworms. 



proboscis wi+k s+yle+ 



V 



SCHISTOSOME LARVA 



POLYCLAD 

 Pie ucloceros-tii. 



Rhabdocoel triclad 



Plo-qios-fortiarn-rrii Bdelloui-a --t in 



ARROW WORM PHORONID 

 ^a<)i + U- Jj-.n Phoroni5-^i-i 



Fig. 33. Unsegmented wonns. Adapted from several authors and drawn from 

 life. 



