692 COLLEGE ZOOLOGY 



cata (Figs. 337-340), which contains a number of sac-like r.nimals 

 that exhibit chordate characteristics chiefly in the immature 

 stages; and (3) the Cephalo chorda, which has but a single 

 genus — Amphioxus (Figs. 341-344). 



A careful study of Amphioxus has brought forth convincing 

 evidence that this animal is really a modified ancestor of the 

 vertebrates. The essential structural characteristics which are 

 possessed in common by Amphioxus and the vertebrates are 

 the presence of (1) a notochord, (2) a dorsal nervous system, 

 (3) a pharynx perforated by gill-slits, and (4) a mid-ventral 

 endostyle. 



If we accept Amphioxus as the invertebrate most nearly allied 

 to the vertebrates, we may then seek for an ancestor of this form. 

 Such an ancestor is supplied by the sea-squirts or Tunicata 

 (pp. 389 to 393). The adult tunicates (Fig. 338) have retained 

 very few of their primitive characteristics, but the larva, as 

 shown in Figure 339, possesses a typical notochord, a neural tube, 

 a series of gill-slits, and an endostyle, which are similar in posi- 

 tion and development to these structures in Amphioxus ; and it 

 seems probable that the adult tunicate once existed as an ani- 

 mal like the larval tunicate of to-day, and that this remote an- 

 cestor was not only the progenitor of the modern tunicates, but 

 was also the direct ancestor of the group to which Amphioxus 

 belongs. 



The search for a vertebrate ancestor more remote than the 

 tunicates leads to a consideration of the marine worm-like ani- 

 mals of the subphylum Enteropneusta. These species, as 

 previously shown (pp. 386 and 389, Figs. 332 and 333), are pro- 

 vided with clearly defined gill-slits, a structure which may be 

 homologous to the notochord of the vertebrates, and four 

 longitudinal nerve-cords of which the dorsal is slightly more 

 pronounced than the ventral and lateral ones. It appears, 

 therefore, that the Enteropneusta may possibly be vertebrate 

 ancestors of an earlier stage than the tunicates. 



We must look to the larvae of the Enteropneusta for the 



