THE CHORDATES 391 



the more primitive vertebrates the digestive tract and the urinary and 

 genital ducts terminate in a common cavity, the cloaca. This opens to 

 the surface by an anus located somewhat anterior to the end of the 

 body, and there is a distinct postanal tall. The respiratory organs are 

 either gills, which lie within the gill slits, or lungs— paired, saccular 

 outgrowths from the floor of the pharynx. The circulatory system is 

 closed, for capillaries connect arteries and veins. Blood is propelled by 

 the action of a muscular heart lying ventral to the digestive tract in an 

 anterior division of the coelom. The excretory organs are kidneys com- 

 posed of numerous kidney tubules (Fig. 5.6 D) that remove both water 

 and excretory products from the blood. In most vertebrates much oi 

 the water is later reabsorbed into the blood. 



1 78. The Origin of Chordates 



Ever since the general acceptance of the theory of organic evolution, 

 man has been interested in the origin of the chordates. But this prob- 

 lem does not have an easy solution, for chordates are a distinctive 

 group separated by a wide morphologic gap from other phyla. 



The segmentation of cephalochordates and vertebrates early drew 

 attention to a possible evolutionary relationship between chordates and 

 the annelid-arthropod stock. Annelids and arthropods are segmented, 

 but they differ from chordates in so many basic characters that this view 

 has been abandoned. Their nerve cord, for example, is not a single, 

 dorsal, tubular cord, but a solid, essentially double cord, lying ventral 

 to the digestive tract. It would be necessary to turn an annelid or 

 arthropod upside down, evolve a completely new nerve cord, and make 

 many other radical transformations in order to derive a chordate from 

 these animals. Intermediate stages in such a transformation are difficult 

 to visualize. Moreover, the urochordates, generally considered to be the 

 most primitive chordates, are not segmented and are a source of em- 

 barrassment to those who would derive chordates from segmented an- 

 cestors. 



Other evidence indicates that the lower chordates may have evolved 

 from the echinoderm-hemichordate stock. The presence in certain hemi- 

 chordates and chordates of pharyngeal gill slits and the unusual tongue- 

 bar that causes the slits to become U-shaped suggest an evolutionary 

 relationship between these groups. Indeed, some authors include the 

 hemichordates as a subphylum of the chordates. Some have concluded 

 that the radial symmetry of echinoderms negates a relationship with the 

 bilaterally symmetrical hemichordates and chordates, but, as we have 

 learned (p. 376), the radial symmetry of the adult, present-day echino- 

 derms has been secondarily superimposed upon a basically bilateral or- 

 ganization. Both the primitive, extinct echinoderms and the early 

 echinoderm larvae are bilaterally symmetrical. Many features of the early 

 development (cleavage, origin of mesoderm and coelom, fate of the 

 blastopore) of echinoderms, hemichordates and chordates are similar and 

 suggest an evolutionary relationship (see section 173). Moreover, there is 

 a closer similarity between the body fluid proteins of the chordates, 

 hemichordates and echinoderms than between those of chordates and 



