in the rock. Worms that do break in escaping from 

 would-be captors, human or animal, almost always 

 replace a missing rear end; and certain species can 

 regenerate a whole worm from any fragment that 

 contains a portion of one of the lateral nerve cords. 

 As in flatworms, the capacity for regeneration goes 

 with the natural capacity of certain species for repro- 

 ducing asexually by fragmentation of the body, es- 

 pecially during warm months. A large specimen of 

 Linens socicilis, which lives gregariously under stones 

 on the American Atlantic coast (or of Linens vei^elns 

 on the west coast), may fragment into six to twenty 

 or more pieces. After transforming into complete 

 worms of smaller size, these grow again and later re- 

 produce sexually. Most though not all ribbon worms 

 are of separate sexes. The eggs are usually laid in 

 gelatinous strings or masses, and the young hatch as 

 juvenile worms. In some species of Linens, in Cere- 

 hratnlns, and in some of their relatives, the egg 

 hatches as a gelatinous, helmet-shaped, free-swim- 

 ming little larva, called a pilidium. It must feed on 

 microscopic organisms and develop further before it 

 takes on the structure of the adult. 



For the most part, ribbon worms are bottom dwell- 

 ers on temperate marine shores, where they burrow 

 in mud or sand or creep about among rocks and sea- 

 weeds between tide marks or in shallow waters. Only 

 a few burrow into the deep-sea bottom, sometimes at 

 depths of forty-five hundred feet or more. Of some 

 570 described species, nearly 200 are found along 

 the Atlantic or Mediterranean shores of Europe. 

 About 100 live on the Pacific coast of North Amer- 

 ica, at least 18 of them identical or very similar to 

 European species. The Atlantic coast of North 

 America has few more than 50 known species, and 

 W. R. Coe, the American authority on nemerteans, 

 thought this was due to the cold arctic current that 

 comes close to the coast as far south as Cape Hat- 

 teras, for many of the missing genera are warm-tem- 

 perate forms. Almost 30 species are described from 

 Japanese shores. In the open seas, chiefly the south- 

 ern parts of the North Atlantic, there are nearly 60 

 gelatinous species that drift or swim slowly far below 

 the surface. They have been brought up from depths 

 ranging from six hundred to nine thousand feet, most 

 from below three thousand feet. Nemerteans are less 

 common in tropical or subtropical seas, but well rep- 

 resented in arctic and antarctic waters, often by the 

 familiar temperate genera: Linens, Amphiporns, 

 Cerebratnlns, and Tetrastemma. 



Perhaps the most cosmopolitan species is Linens 

 rnber, found from Siberia to South Africa. The slen- 

 der, rounded body is 3 to 9 inches long; and different 

 varieties are colored red, green, or brown, any of 

 them difficult to see in natural surroundings, even 

 when one has lifted the stone under which the worm 

 lives. 



Fresh waters, especially in northern latitudes, 

 harbor species of the genus Prosionui. What seems 

 to be a single species, Prostoma rnbrnin, a slender 

 reddish worm less than 1 inch long, can be found 

 in pools and quiet streams in nearly all parts of the 

 United States. It clings to the leaves of aquatic plants 

 and feeds on minute crustaceans, nematodes, and 

 turbellarians. In Europe this genus has also an eye- 

 less variant that lives in caves. 



Land nemerteans are all of the genus Geonemer- 

 tes. The two best-known species are slender, pale in 

 color, and not more than 2 inches long. By exploit- 

 ing the nemertean talent for copious secretion of 

 slime, land nemerteans manage to live along marine 

 shores, in moist earth, or under foliage and fallen 

 logs, in such places as Bermuda, Australia, New Zea- 

 land, and many South Pacific islands. In the Sey- 

 chelles, Geonemertes arboricola occupies the leaf 

 bases of a screw pine (Panckmns) tree, often living 

 high in the tree. 



A common nemertean of the .American Pacific coast, 

 Amphiporns bintaciilatus, that has many relatives on 

 other shores. If is short ( up to 6 inches ) and broad 

 for a ribbon worm. ( Ralph Biichsbainn ) 



131 



