664 EVOLUTION OF ANIMALS Part V 



lar burrowing organ and thus performs a skeletal function. Acorn worms are 

 common on muddy bottoms along both east and west coasts. The pharynx is 

 divided into a dorsal region, containing many pairs of gill slits, and a ventral 

 food passage. 



Urochorda. Members of the Urochorda are called tunicates because of their 

 tunic-Uke covering and sea squirts because they squirt water from the pores 



Fig. 33.2. Diagrammatic side views of the larvae of A, an acorn worm (a 

 hemichordate); B, a starfish; and C, a sea cucumber — all of them minute, 

 nearly microscopic. The black lines represent bands of cilia. In life the digestive 

 tract (stippled) is clearly seen through the translucent body. Until the life history 

 of the acorn worm was known the larvae of acorn worms were taken for star- 

 fishes. This is an example of certain similarities between chordates and echino- 

 derms that has led to the theory that the two groups have a common ancestry in 

 some minute bilaterally symmetrical animals of the ancient oceans. There is also 

 a striking biochemical resemblance. The amino acid, creatine occurs in all verte- 

 brates; among invertebrates it is known only in echinoderms. (Courtesy, Romer: 

 The Vertebrate Body, ed. 2. Philadelphia, W. B. Saunders Co., 1955.) 



of the mantle when disturbed. The larvae, but a few millimeters long, are 

 tadpole-shaped with a notochord in the tail. Appropriately for their free liv- 

 ing, they are equipped with eyes. As they go on developing, they settle front 

 end down on submerged seaweeds and rocks and become permanently at- 

 tached. The tail and notochord waste away and the eyes disappear. These 

 animals are striking examples of evolution gone backward, but their abun- 

 dance shows that they have fitted into a niche in which they have survived 

 with great success. 



Cephalochorda. The Cephalochorda includes the lancelet Amphioxus, the 

 fish-shaped burrowers, two to four inches long, that live in limited zones of sea 

 bottoms all over the world (Fig. 33.3). The basic pattern of these lance- 

 lets resembles that of the vertebrates. The development of the embryo is also 

 a ground plan of vertebrate development and certain studies of it are classics 

 in embryology (Chap. 19). 



Higher Chordates 



SUBPHYLUM VeRTEBRATA 



The animals that attract human interest are most often the vertebrates. In- 

 sects are their chief competitors for attention — the only group that equals 



