208 



MESOZOA THROUGH ENTOPROCTA: 



Figure 12.12 A ribbonworm, Amphiphorus, with proboscis everted 

 (proboscis usually is retracted). 



a pore to hint its presence; 0.2 inch to over 80 feet 

 long; variously red, brown, yellow, green, or white; 

 some are solidly colored, and others are striped or 

 cross-banded; almost all are marine, a very few are 

 fresh water or terrestrial; some are parasitic (Figure 

 12.12). 



The ribbonworms and flatworms are the only 

 multicellular animals of the grade Bilateria (i.e., 

 bilaterally symmetrical animals) that do not possess a 

 body cavity. However, the ribbonworms show much 

 more structural complexity than the flatworms. For 

 example, ribbonworms have a complete digestive sys- 

 tem composed of mouth, alimentary canal, and anus; 

 a circulatory system; and an eversible proboscis that 

 can be retracted into a tubular cavity. 



Ribbonworms are carnivorous, usually free-living, 

 marine, shallow-water bottom dwellers. They are 

 most common in temperate ocean regions under ob- 

 jects, among seaweeds, or in mud, sand, or gravel. 

 Some construct tubes which are stabilized by a mu- 

 cous lining that is secreted and applied by the worm. 

 These and certain others are burrowing forms. A 

 very few species are commensals (in sponges, bivalve 

 mollusks, and tunicates) or parasites (in crabs). 



The sexes are separate. Development of fertilized 

 eggs takes two possible paths in these animals. In 

 some, development is direct, the egg hatching into a 

 miniature adult which simply grows to the adult size. 

 The other path involves the egg's hatching into a free- 

 swimming larva that is reminiscent of a football hel- 

 met. This larva later metamorphoses, or changes 

 form, to the adult worm. Such sexual reproduction 

 may involve the female's retaining the fertilized egg 

 within her body and the young being born alive. 



However, asexual reproduction takes place in some 

 species. During the warm parts of the year these 

 species regularly fragment, each piece regenerating 

 into an adult. 



PSEUDOCOELOMATA: 



ACANTHOCEPHALA, 



ASCHELMINTHES, 



AND ENTOPROCTA 



The spiny-headed worms, cavity worms, and 

 entoproct "moss animals" are the only Bilateria 

 with a pseudocoel, or false body cavity. These three 

 phyla are believed to be closely related to flatworms 

 and ribbonworms because of various similarities 

 (e.g., a ladder-type nervous system) among the five 

 phyla and because it is not a great step from the lack 

 of a body cavity to the presence of a pseudocoel. 



The three phyla probably evolved from somewhat 

 advanced early flatworms. However, the spiny- 

 headed worms present some problem in this inter- 

 pretation because, in spite of the fact that their de- 

 velopmental stages resemble those of tapeworms, 

 adult Acanthocephala are most similar to Aschel- 

 minthes, the cavity worms. Still another problem is 

 presented by the Aschelminthes. One of the classes 

 of that phylum, the rotifers, were probably early 

 offshoots of the aschelminth almost-flatworm an- 

 cestor. This also might be the case with the "horse- 

 hair" worms. The nematodes, gastrotrichs and 

 kinorhynchs, however, seem to be closely related; and 

 the priapuloids appear to be an offshoot of an early 

 ancestral kinorynch. Even if we accept these shaky 

 hypotheses about the relationships among the var- 

 ious classes of cavity worms, we have not solved our 

 final difficulty, the phylum Entoprocta. Although it 

 is here assumed, on good evidence, that entoprocts 

 evolved from an ancestral rotifer, superficially 

 entoprocts resemble ectoprocts (a schizocoel phylum, 

 see p. 213). This resemblance is emphasized by the 

 similarity in the names of the latter two phyla and 

 their both being called "moss animals." However, 

 this similarity seems to be more like that between a 

 butterfly and a bird than an indication of truly close 

 relationship. Entoprocts and ectoprocts have too 

 many fundamental developmental and adult struc- 

 tural diflferences for there to be much likelihood of 

 their close common ancestry. 



