406 Comparative Morphology of Chordates 



unmistakable chordate features of the larva of tunicates and the almost 

 vertebrate nature of Amphioxus compel the conclusion that these ani- 

 mals stand in close relationship to the vertebrates. The close similarity 

 in the general plan of structure of the urochordate larva, Amphioxus, 

 and a "low" vertebrate such as the round-mouthed lamprey eel, 

 Petromyzon, is illustrated in Fig. 315. Compare also the corresponding 

 cross sections of Amphioxus and the larva (ammocoetes) of Petromyzon, 

 shown in Fig. 316. 



Admitting that the "notochord" of the hemichordates is a more 

 or less dubious thing, the pharyngeal clefts and the dorsal nerve-cord, 

 by themselves, are enough to give these animals some claim to a place 

 among the chordates. The resemblance of the hemichordate larva, the 

 tornaria, to the larvae of echinoderms is perplexing. This difficulty, 

 however, would be disposed of if the hemichordate line and the echino- 

 derm line could be traced backward to some remotely ancient ancestor 

 common not only to them but to all the other chordates, the latter, 

 however, in the course of their long and much more progressive careers, 

 having completely eradicated the echinoderm taint from their germ- 

 plasms. This possibility is suggested in the hypothetic "family tree" 

 in the animal kingdom, shown in Fig. 301. The echinoderms and chor- 

 dates at least agree in being deuterostomian. 



The urochordates are of special interest because of their meta- 

 morphosis, which is commonly described as being of a "degenerative" 

 nature since, for the most part, it consists in losing structures which 

 the larva possesses. From the point of view of such a highly superior 

 chordate as man, the metamorphosis is "degenerative" because we 

 regard a life of free and energetic activity as much more respectable 

 than the lazy sedentary habit of the adult sea-squirt. We must admit, 

 however, that the structure of the ascidian is quite adequate for the 

 life which the animal lives. 



The free-swimining tunicates such as Salpa and Doliolum must have 

 an extraordinary history, if current views concerning the urochordates 

 are accepted. According to these views, the primitive urochordates 

 were, as adults, long-tailed and free-swimming chordates similar to 

 the larvae of modern tunicates. In the course of time most of them, the 

 Larvacea being possible exceptions, acquired the "degenerative" 

 metamorphosis. Then, in the course of later time, the descendants of 

 some sessile, saclike forms must have regained the habit of living a 

 free-swimming adult life. But in doing so they did not retain or re- 

 acquire a locomotor tail and notochord. The adults are, in a general 

 way, anatomically similar to sessile ascidians. The main differences 

 are those consequent upon a shifting of axes so that the oral and atrial 

 "siphons" come to point in opposite directions, thus achieving the 



