CHAPTER 20 



The Chordates 



1 74. Chordate Characteristics 



The chordates are perhaps more familiar than the invertebrates de- 

 scribed in the preceding chapters; the phykim includes the back-boned 

 animals or vertebrates— man and his domestic creatures, birds, frogs, 

 fishes, and the like. The Vertebrata, however, is but one subphylum of 

 the phylum Chordata. Two others, the Urochordata and Cephalochor- 

 data, contain less conspicuous, soft-bodied, marine species often col- 

 lectively called the lower chordates. The urochordates are represented 

 by the sea squirts (Molgtila), and the cephalochordates by the lancelet 

 (Amphioxus, Fig. 20.1 C). One may well ask, what do such diverse 

 groups have in common that all are placed in the same phylum? Cer- 

 tainly the adults do not look alike, but at some stage in their life 

 history these animals share three unique features. 



First, a dorsal, longitudinal rod known as the notochord is present 

 in the embryos of all and sometimes in the adults. It is composed of a 



Incurrent 

 - siphon 



Excurrent 

 siphon. 



Oral-, 

 hood 



Myomere — i 



Gelalinou-S 

 ma.trix 



Ca-udal 

 fin 





Gonad 

 '-Meta.pleura.l fold 



Anus 



Ventral fin 

 Atriopore- 



Figure 20.1 . A group of lower chordates. A, The tunicate Molgula, partly buried in 

 sand; B, a portion of the colonial tunicate Botryllus, viewed from above; C, a lateral 

 view of Amphioxus. Molgula is natural size; the others are enlarged. 



383 



