Evolution And Classification

Like echinoderms, chordates are deuterostomes. This similarity provides evidence that echinoderms and chordates likely evolved from a common ancestor. The phylum Chordata is divided into three subphyla: Vertebrata, Cephalochordata (SEF-uh-loh-kawr-DAY-tuh), and Urochordata (YOOR-uh-kawr-DAY-tuh). Members of the subphylum Vertebrata, the vertebrates, constitute more than 95 percent of all chordate species. Members of the other two subphyla live only in the ocean. They are the closest living relatives of the early animals from which all chordates evolved.

Modeling Chordate Characteristics

Materials several colors of clay, toothpicks, masking tape Procedure Build clay models of a lancelet and an adult tunicate by using different colors of clay for the structures shown in Figures 38-11 and 38-12. Make flags using masking tape attached to toothpicks, and use them to identify any of the four major characteristics of chordates that are found in your models. Analysis Which of the major characteristics of chordates are found in the lancelet? the adult tunicate? Which of the four characteristics is shared by both? Why is the tunicate classified as a chordate despite the fact that it has only three of the four chordate characteristics?

Subphylum Cephalochordata

The subphylum Cephalochordata contains about two dozen species of blade-shaped animals known as lancelets. Figure 38-11 shows that lancelets look much like the idealized chordate drawn in Figure 38-10. They retain their notochord, dorsal nerve cord, pharyngeal pouches, and postanal tail throughout their life.

figure 38-11

(a) The lancelet Branchiostoma lanceolatum lives with most of its body buried in the sand. (b) Even as adults, lancelets clearly show all four chordate characteristics.

Mouth

Mouth

Tentacles

Dorsal nerve cord

Tentacles

Intestine

Live Amphioxus

Dorsal nerve cord

Anus

Pharynx with slits

Pharynx with slits

Anus

Tail

Intestine

Segmented muscles

Lancelets live in warm, shallow waters. They wriggle backward into the sand with a muscular tail. Only their anterior end protrudes from the sand. Lancelets have cilia that draw water into the pharynx through the mouth. Food particles in the water are trapped as the water passes through the slits in the pharynx. The food enters the intestine to be digested. The water leaves the body through an opening called the atriopore (AY-tree-oh-POHR).

Lancelets can swim weakly, powered by the coordinated contraction of muscles that run the length of their body. If you look closely at Figure 38-11, you can see that these muscles are arranged as a series of repeating segments. Body segmentation is another common feature of chordates. Recall that annelids and arthropods also have segmented bodies. However, animals in those phyla probably evolved body segmentation independently of chordates.

Subphylum Urochordata

The 2,000 species in the subphylum Urochordata are commonly called tunicates because their bodies are covered by a tough covering, or tunic. Tunicates are also called sea squirts because they squirt out a stream of water when touched. As adults, most tunicates are sessile, barrel-shaped animals that live on the sea bottom. They may be solitary or colonial.

Figure 38-12 shows that tunicates are adapted for filter feeding. Propelled by the beating of cilia, water enters the body through an incurrent siphon, passes through slits in the pharynx, and exits through an excurrent siphon. Food that is filtered by the pharynx moves into the stomach. Undigested material leaves via the anus, which empties into the excurrent siphon.

Tunicates are hermaphrodites. Sperm and eggs are released through the excurrent siphon into the surrounding water, where fertilization occurs.

Adult tunicates bear little resemblance to the idealized chordate shown in Figure 38-10. Although they do have a pouchlike pharynx with slits, they have no notochord, dorsal nerve cord, or postanal tail. Larval tunicates, however, possess all four chordate characteristics, but they lose most of them during metamorphosis.

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