Classification of Cliordates: Protochordata 393 



("proto-") mouth (i.e., blastopore) of the embryo. The second trunk, 

 the Deuterostomians (on the right in Fig. 301), includes the phyla in 

 which the adult mouth is a second ("deutero-") mouth formed at a 

 position opposite that of the primary blastoporal "mouth," which may 

 persist as the adult anus. 



In the succeeding chapters dealing with the various organs of 

 vertebrates, reference will be made to invertebrates whenever com- 

 parison of a vertebrate organ with an invertebrate organ seems rele- 

 vant. It is fair to assume that anyone who uses this book must already 

 have made some acquaintance with the invertebrate phyla. Therefore 

 no further attention will be given here to the characteristics and classi- 

 fication of invertebrates. 



Phylum Chordata 



The most highly distinctive characteristics of vertebrates are these 

 three things: the notochord, well developed in all vertebrate em- 

 bryos; the dorsal position and hollow or tubular form of the central 

 nervous organs; the pharyngeal clefts or pouches, always present 

 at least in the embryo. In these three features, the vertebrates are very 

 far removed from all of the clearly defined phyla of invertebrates. 

 Nevertheless, there are a few animals which are literally invertebrate, 

 in the sense that they have no vertebral column, and yet they more or 

 less definitely possess these three features of vertebrates. Even though 

 they may possess little or no other resemblance to vertebrates, these 

 three characteristics are generally regarded as more significant than 

 the lack of other vertebrate features. These animals are therefore put 

 in the same phylum with vertebrates on the assumption that they must 

 be remote allies of the vertebrates. 



Since the notochord is perhaps the most peculiar of the vertebrate 

 characteristics, it is taken as the basis for the name of the Phylum. 

 Chordata. The notochord (or chorda dorsalis) is defined as a support- 

 ing (skeletal) rod lying dorsal to the digestive tube and ventral to the 

 central nervous organ (neural tube). It is constituted of a histologically 

 peculiar soft tissue — i.e., neither bone nor cartilage — developing in 

 the embryo from, or in close relation to, the middorsal wall (endoderm) 

 of the digestive cavity. 



The nonvertebrate cliordates include three types of animal which 

 are so very unlike that it seems best to set up a separate subphylum 

 for each type. Those of one type, represented by Balanoqlossus (Fig. 

 303), are in most respects wormlike. The structure which may be 

 interpreted as a notochord, in contrast to the vertebrate notochord 

 which extends nearly the whole length of the animal, is confined to 

 the anterior region of the animal. Hence the name of the subphylum, 



