426 THE LANCELET CHAP. 



PRACTICAL DIRECTIONS. 



AMPHIOXUS. 



A, External characters. 



Examine an entire specimen and note : I, the form of the body; 

 2, the continuous median fins dorsal, caudal, and ventral ; 3, the 

 paired lateral fin or mctapleure, extending along the body anteriorly 

 to the ventral fin : 4, the oral hood, anterior and ventral, with its 

 cirri ; 5, the anus, a short distance from the posterior end, just on 

 the left side of the caudal fin ; 6, the median and ventral atriopore, 

 at the junction of the lateral and ventral fins ; 7, the myomeres. 



B. Anatomy. 



The dissection of Amphioxus is too difficult a task for the beginner, 

 and so it is better to proceed as follows : 



I. Place a young specimen for 1 a short time in absolute alcohol, and 

 then transfer to oil of cloves : if the specimen is a small one, it may 

 with advantage be slightly stained first. Then transfer to a hollow 

 slide or, if you use an ordinary slide, support the cover-glass on two 

 small pieces of wood of the thickness of the specimen, and mount in 

 thick balsam : or the preparation may be examined in oil of cloves. 

 (Many details are better seen in very young specimens, not more than 

 half an inch long, which may be purchased ready-mounted. ) Examine, 

 and note in addition to the above points : 



1. The notochord, extending from the anterior to the posterior end of 

 the body, rather nearer the dorsal than the ventral side. 



2. The neural canal enclosing the spinal cord, lying just above the 

 notochord, and having pigment in its wails. 



3. The oral skeleton, consisting c-f a segmented basal bar along the 

 sides of the oral hood, with a rod passing down each cirrus. 



4. The longitudinal series of connective-tissue compartments, which 

 are filled with gelatinous substance, forming the fin-rays. 



5. The elastic, horny rods supporting the gill-bars, and the arrange- 

 ment of the gill-slits in pairs : the primary gill-bars separating the 

 successive pairs, the rods supporting them being forked ventrally ; and 

 the secondary gill-bars between the members of a pair, the rods of these 

 being unsplit ventrally. Note also the horizontal bars connecting the 

 primary and secondary gill-bars and making the walls of the pharynx 

 appear like a mesh work. 



