240 AN INTRODUCTION TO ZOOLOGY. 



chord) is used as a name for a group including both 

 Amphioxus and the Vertebrates, while the Amphioxus 

 itself receives as a group name that of Ceplialoclior- 

 data. This name expresses the fact that it has no 

 true head ; for it refers to the position of the noto- 

 chord, which goes on right to the front of the body, 

 whereas in vertebrate embryos the notochord stops at 

 the middle region of the brain. Many other group 

 names have been given to this little animal, namely 

 those of Acrania (skull-less) ; PharyngohrancMi (pha- 

 rynx-gilled) ; Cirrostomi (fringe-mouthed, because it 

 has filaments round its mouth) ; and Leptocardii 

 (slender-hearted, because its heart is represented 

 merely by a contractile blood-vessel). It is only by 

 courtesy that all these names are put in the plural, 

 since Amphioxus is the sole genus of its type. Few 

 animals have had so many names given them all 

 expressing some special peculiarity. Its generic and 

 specific names are also descriptive of its peculiarities : 

 Amphioxiis, sharp at both ends (Greek a/j,<f)i and 

 o^i;?, sharp), and lanceolatus, lance-shaped, i.e., taper- 

 ing at both ends, both referring to its headless 

 structure. 



At first sight Amphioxus might be described as a 

 very worm-like little fish ; but there are some respects 

 in which its structure is really wonderfully like that of 

 an Ascidian. It is, however, practically bilateral in 

 symmetry, though not quite so. Its breathing ar- 

 rangements are, in the adult state, precisely those of 

 the Ascidian. The water, respired, entering through 



