130 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



13 (14). With one or more girdles or crowns of cilia borne upon the discoidal, 

 platelike, or lobed anterior end; with internal movable chitinous jaws; small or 

 microscopic aquatic forms. Phylum Trochelminthes 



14 (13). Without such crowns of cilia. 15 



15 (27). Body not divided into segments, nor with segmen tally arranged 

 bristles, nor with segmented appendages. 16 



1 6 (17, 20). Wormlike animals without an anus; flattened dorsoventrally, 

 often leaflike or ribbon-like; free-living in water, or parasitic. 



Phylum Platyhelminthes 



17(16,20). Wormlike animals with an anus; without an exoskeleton; heads 

 poorly developed and not distinct from body. 18 



1 8 (19). With a long proboscis inclosed in a sheath lying dorsal to the 

 alimentary canal, capable of protrusion from the anterior end; slender, often 

 very long, flattened to cylindrical worms, mostly marine. Phylum Nemertinea 



19 (18). Without such a proboscis; slender cylindrical worms, pointed at 

 each end, covered by a smooth thick cuticle; water, damp earth, or parasitic. 



Phylum Nemathelminthes 



20 (17, 16). Generally not wormlike, with an anus; often provided with 

 an exoskeleton, consisting of calcareous shells of one or more pieces, or cal- 

 careous or horny cases or tubes, or a gelatinous secretion; if naked, with much 

 more definite heads than under 16 and 17. 21 



21 (22). Colonial animals, sessile, fastened in cases, tubes, or on the surface 

 of gelatinous masses; provided with a circular or horseshoeshaped crown of 

 ciliated tentacles; small, usually microscopic. 



Phylum Molluscoidea; Class Bryozoa (Polyzoa) 



22 (21). Not colonial, nor microscopic, exoskeleton in the form of a cal- 

 careous shell of one or more pieces, or absent. 23 



23 (24). With definite heads, sometimes bearing tentacles armed with 

 suckers; shell, if present, not bivalve. Phylum Mollusca 



24 (23.) Without definite heads; shell bivalve. 25 



25 (26). Halves of the shell dorsal and ventral in position; nearly always 

 fastened by a stalk protruding between the valves at their hinge. 



Phylum Molluscoidea; Class Brachiopoda 



26 (25). Halves of the shell lateral; never with such a stalk, although 

 sometimes sessile. Phylum Mollusca 



27 (13). Body divided into segments, or with segmentally arranged bristles 

 or internal organs, or with segmented appendages. 28 



28 (33). Without jointed appendages; wormlike. 29 



29 (30). Remarkably elongated, flat, tapelike worms, without mouth, anus, 

 or digestive tract; head and anterior segments very small; segments very 

 numerous, increasing greatly in size posteriorly, where they drop off when ripe; 

 always internal parasites. Phylum Platyhelminthes 



