CHAPTER 19 



The Phyla 

 Hemichordata and Echinodermata 



Hemichordates and echinoderms are sedentary or slow-moving inhab- 

 itants of the ocean floor. Most of them feed on debris and microscopic 

 organisms, although a few echinoderms are predaceous. Both phyla are 

 entirely marine. They range from the shoreline to the ocean depths, 

 and from the tropics to the poles. Echinoderms are conspicuous and 

 common everywhere, but the hemichordates are seldom noticed, al- 

 though they may be locally abundant in the sand and mud. Echinoderms 

 have a predominantly radial symmetry which is not as well developed, 

 however, as that of the coelenterates. The hemichordates are of special 

 interest to the zoologist because they show affinities with both the 

 echinoderms and the chordates. 



164. The Phylum Hemichordata 



Hemichordates are bilaterally symmetrical animals with a body di- 

 vided into three regions (Fig. 19.1): the proboscis, the collar and the 

 trunk. The proboscis contains an anterior projection of the gut, the 

 stomochord. The collar has a well developed dorsal collar nerve, and nu- 

 merous gill slits open into the pharynx along the sides of the trunk. 



Each body region contains a separate portion of the eucoelom. The 

 portion in the proboscis (coelomj) opens to the outside through one 

 or two dorsal pores. The muscular proboscis can expand or contract, 

 flushing sea water in and out of its cavity. The portion in the collar 

 (coelomo) opens to the outside through a pair of lateral pores; it can 

 also be filled and emptied with seawater. The third portion (coelomg) 

 forms a typical body cavity in the trunk, lying between the viscera and 

 the body wall. 



The phylum is divided into two classes. The larger class is the 

 Enteropneusta which includes the wormlike form used in Figure 19.1 C. 

 Its sixty species vary in length from one to one hundred inches. The 

 smaller class, Pterobranchia, includes a few minute species, some of 

 which are colonial. In the pterobranchs the collar with its coelomic 

 cavity is expanded dorso-laterally (Fig. 19.2) as a pair of branched 

 tentacles used for gathering food. Tlie trunk is folded so that the anus 

 lies just behind the mouth. Despite their small size the pterobranchs 



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