THE PHYLUM CHORDATA 



FiGxiRE 52. Diagrams of the Tor- 

 NARiA Larva of Balanoglossus (A), 



THE BiPINNARIA LaRVA OF A StARFISH 

 ( B ) , AND THE AURICULARIA LaRVA OF 



A Sea Cucumber ( C ) . ( After Delage 

 and Herouard, from Romer, "The Ver- 

 tebrate Body," 2nd Ed., W. B. Saun- 

 ders Co., 1955. ) 



terostome characters. Thus it appears that the objections to the anneHd 

 theory outweigh its merits. 



The Echinoderm Theory. And so we are left with the echinoderm 

 theory, based as it is upon conformity of both echinoderms and primitive 

 chordates to that set of embryological characters described for Deuterosto- 

 mia generally. The theory received its major impetus from the discovery 

 that the Tornaria larva, originally described as a starfish larva, was actu- 

 ally that of Balanoglossus, a hemichordate (Figure 52). At that time (the 

 last quarter of the nineteenth century), the hemichordates were generally 

 regarded as the most primitive subphylum of the Chordata. This is re- 

 inforced by serological and other biochemical evidence. Perhaps the 

 majority of zoologists still accord them that position, but specialists on 

 the group are more inclined to feel that a separate phylum, Hemichordata, 

 should be recognized. Whether this will decrease the prestige of the 

 echinoderm theory of descent remains to be seen. It is generally agreed 

 that the phylum Hemichordata must be closely related to the phylum 

 Chordata, even if it be conceded that the two phyla are distinct. At any 

 rate the echinoderm theory now has more support than does any other 

 theory of chordate origin, but few would care to claim that it is securely 

 established. 



MAJOR DIVISIONS OF THE PHYLUM CHORDATA 



The phylum Chordata includes three subphyla, the Urochordata or tuni- 

 cates; the Cephalochordata, including Amphioxus, the favorite of most 

 elementary zoology texts; and the Vertebrata, much the most important 

 subphylum, including as it does the dominant animals of land, sea, and 

 air. The first two, together with the Hemichordata, have been exhaustively 

 studied for evidence of the origin of the vertebrates. Morphologically, the 

 tadpole larva of the tunicates is well described by the three fundamental 

 characters of the chordates, that is, it has a dorsal nerve cord, a notochord, 

 and a pharynx perforated by gill slits, and Berrill believes that the verte- 

 brates arose from ancient tunicates. In the adult, however, the nerve cord 

 and the notochord degenerate. The gills, which are a feeding mechanism 

 rather than respiratory structures, become extremely highly developed. 

 Adult tunicates are generally sessile and there are three well marked 

 classes in the subphylum. Amphioxus was long regarded as an ideal 

 "ancestor" because of its beautiful simplicity and its organization around 

 the basic chordate characters. It has since become apparent, however. 



157 



