. Principles of modern biology. Biology. The Animal Kingdom - 665 vertebrates evolved and how the vertebrate classes are related to each other. But the earlier record is very poor. Apparently early chordates did not possess skeletal parts suit- able for fossilization and we are left to specu- late upon other clues including the study of the few surviving lower chordate forms. Subphylum h The Hemichordata. The acorn worms, or tongue worms (Fig. 32-36), are the principal representatives of the Hemichor- data. Typically, such animals possess a stiff tough proboscis (for burrowing), behind which l
. Principles of modern biology. Biology. The Animal Kingdom - 665 vertebrates evolved and how the vertebrate classes are related to each other. But the earlier record is very poor. Apparently early chordates did not possess skeletal parts suit- able for fossilization and we are left to specu- late upon other clues including the study of the few surviving lower chordate forms. Subphylum h The Hemichordata. The acorn worms, or tongue worms (Fig. 32-36), are the principal representatives of the Hemichor- data. Typically, such animals possess a stiff tough proboscis (for burrowing), behind which lie an acorn-shaped collar and an elongate body (Fig. 32-36). Pharyngeal gill slits are present in the anterior section of the body, but the notochord is short, confined to the proboscis region and not exactly similar, structurally, to the notochords of other chor-. Fig. 32-36. A hemichordate animal: the acorn worm (tongue worm), Dolichoglossus. Perhaps the extinct stock from which the hemichordates arose likewise gave rise to vertebrates. (From General Zoology, by Miller and Haub. Holt, Rinehart and Winston, Inc.) dates. Moreover, there is a ventral nerve cord in addition to the dorsal one; and only the anterior section of the dorsal nerve cord is hollow. The larval stages of Echinodermata and Hemichordata are very closely similar. This may indicate origin from a common early stock. Subphylum 2: The Urochordata. The Tuni- cata (Fig. 32-37), which collectively comprise the Urochordata, can be recognized as Chordata only during the larval stages. The larval tunicate (Fig. 32-37) is an active swim- mer, which tends to resemble a good-sized tadpole. Sectioned specimens display a dor- sally placed, hollow nervous system, fairly distinct gill slits, and a notochord. The noto- chord is typical, except that it lies mainly in the tail region. As adults, however, most tunicates become attached to the bottom, develop a tough muscular mantle or tunic around the body, and lose all resemblan
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