searching them out. Perhaps this is why almost all 

 the known species of acoels have been described 

 from temperate or arctic Atlantic waters close to the 

 haunts of most biologists, or in the Mediterranean or 

 other seas that connect with the Atlantic. That part 

 of the Atlantic known as the Sargasso Sea is the home 

 of Amphiscolops sargassi. which lives on the floating 

 sargassum seaweed. Tropical or Pacific species are 

 usually drifting forms picked up in nets towed from 

 boats. Two shore species are known from Monterey 

 Bay, California; but again, this is a base for sharp- 

 eyed biologists. 



Most acoels are white or drab in color, but one of 

 the most celebrated species, Convoluta roscoffensis, 

 is a beautiful rich green from the green algal cells 

 that pack the elongated body (Plate 36). This spe- 

 cies of Convoluta is named for Roscoff, France, the 

 little lobster-fishing port where the University of 

 Paris maintains the largest of its several marine sta- 

 tions. It occurs also on certain sandy beaches in Brit- 

 tany and Normandy, always in dense concentrations 

 of many thousands or millions of tiny worms. The 

 patches look like splashes or streaks of fresh dark 

 green paint on the wet sand laid bare by a receding 

 tide. Concentrated only where they can be continu- 

 ously wetted by rivulets of draining water throughout 

 the low-tide period, the worms lie moist and glisten- 

 ing, displaying their green cells to the sun. Then as 

 the tide returns and the first waves roll in, the green 

 patches erase themselves in an instant. The worms 

 sense the distant wave shock and dig rapidly below 

 the surface. Twice in twenty-four hours, in rhythm 

 with the tides, the worms rise to the surface and later 

 sink below, keeping beyond the reach of pounding 

 waves yet providing exposure to light for the green 

 cells. 



The young convolutas are white, like most acoels, 

 but soon they become infected with green cells, which 

 appear to be derived from little green flagellates that 

 may also be found living free in the sand. At first the 

 convolutas continue to feed voraciously on small or- 

 ganisms, and the plant-animal bond seems no diff'er- 

 ent from what we saw earlier in protozoans and coel- 

 enterates. The photosynthetic cells utilize gaseous 

 and especially nitrogenous animal wastes, and this 

 benefits the animal also by speeding its chemical 

 turnover. As the convolutas mature something hap- 

 pens that suggests the relationship has become un- 

 balanced. The worms stop feeding and begin to di- 

 gest the green cells, eventually dooming both part- 

 ners, though not before the convolutas have laid 

 eggs in the sand and ensured a new generation. 



Many acoels have no eyes and depend on general 

 sensitivity of the body to light; some have on the 

 head two pigmented spots that overlie nervous tissue 

 sensitive to light. Convoluta roscoffensis has two such 

 orange-pigmented eyes, and between them lies an 



otocyst, a tiny balancing organ like those seen in 

 many coelenterates. It shows as a golden dot in the 

 center of the head on several of the worms in 

 Plate 36. 



THE RHABDOCOELS 



A straight and unbranched digestive cavity distin- 

 guishes the little rhabdocoels ("rodlike cavity"), and 

 it can be readily discerned through the transparent 

 and usually colorless body wall. These are very small 

 worms, microscopic or in most cases measuring less 

 than Va of an inch. Of elongate shape, they may be 

 plump or slender, and usually are clothed with short 

 cilia. Most have a pair of pigmented eyes at the head 

 end. Rhabdocoels are common in all fresh waters and 

 on marine shores, especially on sandy or muddy bot- 

 toms. A few are restricted to caves or hot springs or 

 manage to live in moist places on land. Microstonmm 

 occurs in both fresh and salt waters. A fresh-water 

 species common in the eastern United States and in 

 Europe is known for its armory of stinging cells, ob- 

 tained from the hydras on which it feeds. When 

 Microstonmm undergoes asexual division of the body 

 the parts do not separate at once, so that after several 

 successive divisions there results a chain of connected 

 subindividuals, each with its own mouth. 



Formerly lumped with the rhabdocoels are the 

 similar, though generally a little larger, alleocoels. 

 These are now placed in a separate order. 



THE TRICLADS OR PLANARIANS 



Called triclads from their "three-branched" diges- 

 tive cavity, or planarians because they are usually 

 "level" or flattened, this group of flatworms is the 

 most familiar because of the extensive use to which 

 certain fresh-water species have been put in teaching 

 and in research, as was mentioned earlier. Especially 

 when the thin body wall is unpigmented, one may be 

 able to see the three main branches of the digestive 

 cavity, each with numerous side branches. From a 

 point not far from the middle of the body, one main 

 trunk extends forward into the head, and the other 

 two extend backward on either side of the elongated 

 body, which tapers to the rear. The mouth is on the 

 under surface, near the middle of the body, and 

 through the mouth triclads can protrude a long, mus- 

 cular feeding tube or pharynx. 



The fresh-water planarians, the marine ones, and 

 those that live in moist places on land, belong to dif- 

 ferent suborders. Such correspondence between hab- 

 itat and classification, were it more general, would 

 greatly simplify the text of a book such as this one. 

 Unfortunately it is very unusual among animals, as is 

 pointed out by Libbie Hyman, the American author- 

 ity on flatworms, in that volume of her Treatise on 

 Invertebrates that deals exhaustively with the group. 



Fresh-water planarians favor temperate waters 



i 



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