156 ANIMAL BIOLOGY 



spends to both the brain and spinal cord in ourselves; 

 but how remarkably simple is the brain in this low crea- 

 ture compared with the brain of even a fish or a frog ! A 

 pigment spot in the wall of the brain marks the position 

 of a rudimentary eye. The heart is represented by a 

 tubular blood vessel on the ventral side which propels the 

 blood forward; the blood then flows through the vessels 

 in the gills, where respiration is effected and then back- 

 ward in a dorsal vessel. 



Although Amphioxus has no limbs or skull and but a 

 suggestion of a brain its vertebrate characters are un- 

 doubted, and it matters little whether it is classed just 

 within or just without the vertebrate group so long as we 

 recognize its affinities. It is now customary to group the 



FIG. 126. Amphioxus; c, nerve cord; gs, gill slits; i, intestine; /, liver; 

 m, mouth; n, notochord. 



vertebrates, Amphioxus, the tunicates, Balanoglossus 

 and a few other forms in a large phylum called Chordata, 

 the members of which all possess a notochord at some 

 period of their life history. From what group of the 

 invertebrates the chordates took their origin is a question 

 to which the few intermediate groups which now exist do 

 not enable us to give a certain answer. 



The lowest true vertebrates, if we except Amphioxus, 

 are found in the class of Cyclostomes, or round mouths, 

 which include the lampreys and the hag-fishes. These 

 animals have long eel-like bodies without any traces of 

 limbs. There is a cartilaginous skull enclosing a well 



