PHYLUM CHORDATA 



321 



"notochord" is not a true notochord, but is 

 what they call a stomocord. However, as ex- 

 plained in the footnote (p. 315), this is still 

 an open question. 



The acorn or tongue worms are the com- 

 mon names by which the hemichordates are 



known. They are soft-bodied animals and 

 most of them live in shallow water along the 

 seashore. Some species have persistent and 

 unpleasant odors. The external features of 

 one are shown in Fig. 203. 

 Three regions may be distinguished: a 



Proboscis 



Gill slits 



Anus 



Figure 203. An acorn worm, Saccoglossus (Dolichoglossus) kowalevskii, a species that lives on 

 sand flats from Massachusetts Bay to Beaufort, N.C. Natural size about 7 inches long. 



proboscis, a collar, and a trunk, which make 

 up most of the body. Paired lateral gill slits 

 are present in the anterior part of the trunk. 

 Figure 204 shows diagrammatically the prin- 

 cipal internal structures of another species. 

 The mud or sand in which the animals live 

 is taken into the mouth and forced slowly 

 through the digestive tract, where nutriment 

 is extracted from the organic matter con- 

 tained in it— a process similar to digestion 

 in the earthworm. Respiratory, circulator)', 

 and nervous systems are present. The sexes 

 are separate. In some species each egg de- 

 velops into a free-swimming larva called a 

 tornaria. The resemblance of the tornaria 

 to the larvae of echinoderms is quite striking 

 and has led to one hypothesis of the origin 

 of the vertebrates (Chap. 36). 



Subphylum Urochordata 



The tunicates all live in the sea. They are 

 either free-swimming or attached; they are 

 widely distributed and occur at all levels 

 from near the surface to a depth of over 



three miles. They range in size from about 

 YioQ inch to over a foot in diameter. Some 

 are brilliantly colored. The adult in some 

 species (Fig. 205) is saclike and has received 

 the common name "sea squirt" because 

 when irritated it may eject water through 

 two openings in the unattached end. The 

 term Tunicata was formerly applied to mem- 

 bers of the group on account of a cuticu- 

 lar outer covering known as a tunic or 

 test. 



The chordate characteristics of tunicates 

 were not recognized until the development 

 of the egg and metamorphosis of the larva 

 were fully investigated. It was then dis- 

 covered that the typical larva (Fig. 206), 

 which is about V4 inch long and resembles a 

 frog tadpole, possesses ( 1 ) a distinct noto- 

 chord, (2) a dorsal neural tube in the tail 

 enlarging in the trunk and ending in a 

 vesicle, which is considered the forerunner 

 of the brain of the vertebrates, and (3) a 

 pharyngeal sac which opens to the exterior 

 by innumerable ciliated gill slits. The tail 

 propels the larva fonvard by lateral strokes. 



