194 AMPHIOXUS. 



atrial cavity by dehiscence of the proper wall of 

 the ovary and of the atrial membrane. The dis- 

 charged ova, together with the ovaries, form a 

 bulky mass, which causes great distension of the 

 atrial cavity and distortion of the pharynx and other 

 organs. 



The ova, which measure -^^ inch in diameter, 

 sometimes pass through the gill-slits into the 

 pharynx, and escape through the mouth ; in other 

 cases they are discharged through the atrial pore. 



2. The male. 



The testes are similar in form and position to the 

 ovaries ; and the spermatozoa, when ripe, are dis- 

 charged, like the ova, into the atrial cavity, from 

 which they escape by the atrial -pore. 



II. DISSECTION OF AMPHIOXUS. 



Take an adult specimen which has been macerated for 

 about a day and a half or longer in 20 per cent, nitric acid ; 

 and dissect it as described below. 



Specimens which have been preserved in alcohol serve 

 perfectly well for this purpose. The effect of the nitric acid is 

 to soften and loosen the connective tissue, and so render the 

 dissection much easier. 



1. External Characters. 



Place the animal under water in a sttmU dissecting dish. 

 Determine all the external characters, as described on p. 181. 



Lay the animal on its left side, and fix it down by two 

 small pins passed through the body slightly nearer the ventral 

 than the dorsal border, one pin passing through the buccal 

 cavity about one-sixteenth of an inch from the anterior end ; 

 the other through the tail, behind the anus, and about one- 

 eighth of an inch from the hinder end of the body. 



2. The skin. 



With needles or fine forceps strip off the shin from the 



