136 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



MORPHOLOGY AND PHYSIOLOGY. 



1. External Characters (Fig. 38). The scientific and popular 

 names of Amphioxus are alike derived from the fact that the 

 flattened body is pointed at both ends. There is no distinct 

 division into head, trunk, and tail, as is the case in a fish, and, 

 at the first glance, it does not seem easy to say which is the 

 anterior and which is the posterior end. The former, however, 

 is recognizable on a cursory examination by the presence of the 

 ventral mouth, an oval opening bordered by numerous stiff ciliated 

 processes, the buccal cirri. Further back along the ventral surface 

 is seen another median aperture, the atriopore, situated on a pro- 

 minent papilla. It is the outlet of a large atrial cavity, by which 

 the perforated pharynx is surrounded. A thin, laterally flattened 

 fin runs from the mouth round the front end of the body along 

 the dorsal surface, round the posterior end, and forwards along 

 the ventral surface as far as the atriopore. About the middle 

 of this ventral section of the fin a small opening, the anus, is to 

 be found on the left of the median line. This is one of several 

 particulars in which Amphioxus deviates from strict bilateral 

 symmetry. The fin is rather larger round the posterior end, 

 constituting a caudal fin, while that part of it which runs along 

 the upper side of the body in front of this is known as the dorsal 

 fin, and the part between anus and atriopore as the anal fin. 

 All these are perfectly continuous. The ventral surface between 

 mouth and atriopore is broad, gently convex, and marked by a 

 series of longitudinal ridges. It is bounded on each side by 

 a longitudinal fold (metapleural fold), the lateral fin, which unites 

 with its fellow just behind the atriopore, at the point where the 

 anal fin begins. 



Amphioxus is a segmented animal, and this is indicated exter- 

 nally by a number of > shaped lines on the sides of the body, 

 corresponding to a division of the lateral muscles of the body into 

 muscle-segments (myomeres, myotomes) which in Amphioxus lanceo- 

 latus, the species commonly used in laboratories, are 61 or 62 in 

 number. The atriopore corresponds to the 36th myomere, while 

 the anus is situated between the 51st and 52nd. 



There is a small ciliated pit (? olfactory) on the left side of the 

 head above the anterior end of the mouth. 



