THE CEPHALOCORDA . 175 



body, but in front of this the ventral surface is flat, marked 

 with several longitudinal wrinkles, and bounded on either side 

 by a fin-like expansion of the integument, called the meta- 

 pleur. The ventral ends of the myotomes only reach down 

 to the metapleural folds. At about the level of the seventh 

 myotome, the metapleur of each side is continued into a 

 triangular flap fringed on its free margin with eleven 

 tentacle-like processes, the buccal cirrhi. The flap of the 

 right side is continuous with the anterior median fin. 

 These two flaps form a hood enclosing a wide but shallow 

 funnel-shaped cavity, often described as the buccal cavity, but 

 it will be best to call it the vestibule. The mouth is situated 

 at the posterior end of the cavity at the bottom of the funnel. 

 It should be clearly understood that the wide aperture of the 

 vestibule is not to be regarded as the mouth. The buccal 

 folds, as the triangular flaps are called, are formed relatively 

 late in larval life, some time after the true mouth, which they 

 enclose, has been established. The other external openings 

 are the anus, the atriopore, and the so-called olfactory pit. 

 The anus opens on the fifty-second myotome, not in the 

 middle line, but on the left side of the body. The atriopore, 

 which must not be mistaken for the anus, is a widish, nearly 

 triangular opening on the ventral surface on the level of the 

 thirty-sixth myotome, just in front of the anterior end of the 

 ventral fin. It is the exhalant aperture of a large chamber, 

 the atrium, which surrounds the lower part of the anterior end 

 of the body behind the mouth, and receives the water dis- 

 charged through the numerous gill slits which perforate the 

 body-wall in that region. The "olfactory pit" is a small 

 conical depression situated on the left side of the median 

 dorsal fin on a level with the first myotome. Nothing is 

 known about its function, but it is associated with an important 

 embryonic aperture called the neuropore. Just in front of the 

 olfactory pit is a small black pigment spot seated on the 

 anterior end of the nervous tube. This spot is called the 

 eye ; it has no lens, cornea, or retina, and is nothing more 

 than a collection of pigment granules, but as Amphioxus is 

 extremely sensitive to light, it is concluded that this spot is 

 specially sensitive. Similar pigment spots occur at intervals 

 along the length of the neural tube. 



Before proceeding to the study of the internal organs, it is 



