THE PHYLUM CHORDATA 35 



cirri as ^is, he applied to it the name of Branchiostoma (gill- 

 mouthed), a name still retained by experts in nomenclature. The 

 name Amphioxus, however, though given a year or so later by 

 Yarrel, has been in general use for so long that it will be difficult 

 to displace. 



Amphioxus has been studied extensively for over a century and 

 few details of its structure or development have escaped analysis. 

 It has come to be rather generally believed, following Willey, that 

 this form, "though specialized in some particulars and degenerate 

 in others, represents a grade of organization not far removed from that 

 of the main line of early chordate ancestors." Whether Amphioxus 

 is primitively simple or secondarily simplified by degeneration, the 

 fact remains that in its structure and development it shows in a 

 strikingly diagrammatic way the essential characters of the chordates. 

 On this account Amphioxus has become a favorite laboratory type 

 throughout the educational institutions of the civilized world and is 

 studied annually by thousands of students. 



The Cephalochordata consist of several closely similar speclos 

 grouped into two genera, Branchiostoma (Amphioxus) and Asymme- 

 tron. The group is cosmopolitan in distribution, occurring almost 

 everywhere in the temperate zone where sloping sandy sea-shores 

 exist. This wide distribition and slight variability are taken by some 

 writers to mean that the group is extremely archaic. 



The writer is inclined to look upon Amphioxus as a form which has 

 lost through sedentary life most of its head parts .and is therefore 

 partially acephalic, a view that is based on the following considera- 

 tions. Typically, the chordate notochord runs only up to the head 

 proper, and the fact that in Amphioxus the notochord extends to the 

 anterior end of the body (Fig. 8) may mean that the head is degener- 

 ate has retreated from the anterior end.~~This interpretation of Am- 

 phioxus is in accord with the fact that the tunicate larva has a much 

 better head than has Amphioxus. Strictly speaking, Amphioxus is 

 not headless; it has merely a reduced or degenerate head which does 

 not extend in front of the trunk, but has come to lie back of the most 

 anterior parts of the trunk. In the tunicates also the reduced 

 brain lies posterior to the mouth and parts of the pharynx. The 

 ancestral Amphioxus probably had a head with paired eyes, otic 

 vesicles, and a considerably larger brain than is found in the modern 

 representatives. Possibly also the pharynx is more specialized than 



