GENERAL ZOOLOGY 



the only skeleton. In some of these forms the notochord may persist only 

 through the larval stages. The Craniata include the chordates with skulls 

 and with vertebrae. In these the notochord always appears in early develop- 

 ment, but it is subsequently more or less replaced by vertebrae, which form 

 around it during the differentiation of the familiar cartilaginous or bony 

 endoskeleton. The subdivisions of the Acraniata and Craniata, with their 

 general characteristics, are outlined on pages 541 and 542. 



It now appears probable that the fossils known as Graptolites represent 

 extinct members of the phvlum Hemichordata. If this interpretation of the 

 evidence is correct, the fossil record of the hemichords extends from the 

 Cambrian period (see Fig. 20.1, p. 617). The first recognizable remains of 

 chordates appear as fossils in the Ordovician period, but it must be assumed 

 that earlier types of chordates, lacking skeletal parts that could be preserved 

 as fossils, existed long before this time. From the Devonian onward, the fossil 

 record of the Chordata is more comprehensive than that of any other phylum. 

 The study of the evolutionary history of the phylum Chordata is in part the 

 study of the remote ancestry of mankind, since man is a chordate. 



In this chapter we shall discuss briefly the phylum Hemichordata and its 

 probable relationship to the Chordata, proceeding to a consideration of the 

 chordates themselves. In earlier chapters, principles of vertebrate structure 

 and function have been discussed so extensively that we shall now consider 

 in detail only the chordates that are not vertebrates. For the vertebrates, 

 we present in this chapter a survey of the principal types and their evolu- 

 tionary history. In considering this history, we shall be anticipating the 

 subject matter of a later chapter which deals with evolution in general. 

 However, in dealing with the various invertebrate phyla we have outlined 

 their apparent phylogenetic development and evolutionary history, and the 

 vertebrates will be similarly treated. When we speak of "ancestors" and "de- 

 scendants," as indicated from the fossil record, it will be with the understand- 

 ing that the evolutionary interpretation is to be accepted as the most reason- 

 able explanation of the facts. The specific evidence for evolution in general, 

 with further examples of the evolution of vertebrates, will follow in Chapter 20. 



In view of the frequent references to geologic time and to the fossil record 

 which it is necessary to make in the present chapter, it is important that 

 Figure 18.8, page 554, as well as Figure 20.1, page 617, be studied carefully 

 and kept in mind. It is also important that the characteristics of the two 

 phyla and their subdivisions be noted, as shown in the following classification. 



PHYLUM HEMICHORDATA 



Class Graptolithina (e.xtinct). Colonial organisms, consisting of chains of zooids 

 produced by budding from a common stolon, often attached to a bladder-like float. 

 Rapidly evolving forms; the fossil remains are interpreted as representing five 

 orders. Became extinct during the Carboniferous period. Diplograptus and many 

 others (Fig. 18.2). 



540 



