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-fold or metapleure, ex- 

 tending along the body anteriorly to the ventral fin ; 4, the 

 oval 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 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 : 



1. Place a young specimen for 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 olorsal than the 

 ventral side. 



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

 above the notochord, and having pigment in its walls. 



3. The or al skeleton, consisting of 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 compart- 

 ments, which are filled with gelatinous substance, forming 

 the fin-rays. 



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

 the arrangement 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 



