Phylum Hemichordata Pterobranchs And Acorn Worms

Another unusual group ofliving marine deuterostomes may offer further clues about the origin of the chor-dates. These are the hemichordates, a phylum that in-

Phylum Hemichordata: Pterobranchs and Acorn Worms 5

eludes two superficially very different kinds of marine animals. The first, the pterobranchs such as Cephalodis-cus (Figure 1.4(a,b)), are small animals that live in loose colonies on the sea-bed in the southern hemisphere and in equatorial waters. Cephalodiscus has a plate-like head shield, a collar with five to nine pairs of feeding arms, and a sac-like trunk perforated by a pair of gill slits and containing the gut and gonads, and the body ends in a contractile stalk. Cilia on the arms produce a feeding current, and food particles are captured by mucus on the arms, while water passes out of the pharynx through the gill slits. The animal lives in or around a group of horny tubes that the colony has constructed, and it attaches itself inside these tubes by means of a sucker on the end ofthe stalk.

The second hemichordate group, the acorn worms, or enteropneusts, such as Saccoglossus, are worm-like animals varying in length from 20 mm to 1.8 m. They

Phylum Saccoglossus
Fig. 1.4 Typical hemichordates: (a) the pterobranch Cephalodiscus,internal anatomy and (b) mode oflife; (c) the enteropneust Saccoglossus,mode oflife and external anatomy. (Modified from Jefferies, 1986.)

live in burrows low on the shore in Europe and elsewhere. Saccoglossus (Figure 1.4(c)) has a long muscular proboscis that fits into a fleshy ring or collar behind. The mouth is placed beneath this collar, and seawater and sand are pumped through the gut and expelled through an anus at the posterior end of the body. The long body is pierced by small holes at the front end, probably equivalent to the gill slits of Cephalodiscus, sea squirts, and amphioxus.

It was suggested that the Pterobranchia and En-teropneusta should be regarded as two separate, but closely-related, groups (Peterson, 1995), although more recent molecular work (Winchell et al., 2002) concurs with morphological data (Smith et al.,in press) that Hemichordata is indeed a valid phylum, and more closely related to echinoderms than to chordates. Hemichordates do not have a notochord at any stage, but they possess gill slits, as in chordates, and giant nerve cells in the nerve cord of the collar region that are probably equivalent to similar nerve cells in amphioxus and primitive vertebrates. Both pterobranchs and en-teropneusts share morphological characters indicating monophyly of the Hemichordata, such as the stomo-chord (an anterior buccal tube on the dorsal part of the pharynx) and mesocoelomic ducts.

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