FORM. 



411 



mouth projecting. Into this diatoms and other small 

 organisms are sucked. At times, and especially in the 

 evening, the adults sometimes start up and swim about, 

 but they are always less active than the larvae. The early 

 larvae are indeed pelagic. 



Form. 



The body, which rarely measures as much as two 

 inches in length, is pointed at both ends, as the names 

 suggest, and bears a dorsal and an anal cuticular fin, con- 

 tinuous around the tail. When alive the animal appears 

 much plumper than the spirit specimens, and is translucent 

 with a faint flesh colour. The muscles are arranged in 

 sixty-two segments, or myotomes, readily visible externally. 

 There are three unpaired apertures (a) the mouth is 

 median and ventral, and overarched by a pre-oral hood, the 

 edges of which are fringed with tentacle-like cirri ; (b) the 

 atriopore opens to the exterior in myotome thirty-six, and 



FIG. 132. Lateral view of Amphioxus. 

 LANKESTER.) 



Note the notochord running from tip to tip. 



/., Tentacular cirri; G., reproductive organs; a.p., atriopore; 

 a., position of anus ; 40 and 62, indicate number of myotomes. 



gives exit to the water which enters by the mouth ; (c) the 

 anus is ventral and slightly to the left side, behind the 

 atriopore, but some distance from the posterior end of the 

 body. Along the back there is & median fin, which is 

 continued around the tail, and along the ventral surface as ' 

 far as the atriopore. In front of this region the ventral 

 surface is flattened, the flattened area being fringed on 

 either side by a slight fin-like " metapleural" fold. These 

 are continuations downwards of the walls of the atrial or 

 branchial chamber, which, extends from behind the mouth 

 to the atriopore, and into which the gill slits of the pharynx 

 open in the adult. 



62 



