36 PHYLUM CEPHALOCHORDA. 



stage lasts for about three months, during which the larva 

 swims freely usually at a considerable depth by the contrac- 

 tion of its body, and acquhes the adult form and habits. The 

 principal changes which take place relate to the formation of 

 the gill -slits, of the preoral hood, and symmetrical adult mouth, 

 and of the atrial cavity. 



The mouth acquires a gigantic size and forms a most con- 

 spicuous object on the left side of the body (Figs. 21, 22). The 

 gill-slits are formed successively on the ventral middle line to the 

 number of fourteen.* All of these except the last few shift 

 soon after their formation on to the right side. They corre- 

 spond in number to the myotomes in the part of the body 



nrr th. 



Fig. 20. — Amphioxus larva of about thirty-six hours from the left side, when the preoral pit- 

 mouth, first gill-slit and anus are established (from Korsehelt and Heider after Hatschek); 

 c larval caudal fln ; ch notochord ; en neurenteric canal ; d alimentary canal ; /: right preoral 

 sac ; k club-shaped gland, which has acquired an opening to the exterior on the left side 

 ventral to the mouth ; ks first gill-slit ; m mouth ; mr nerve tube ; np neuropore ; sv sub- 

 intestinal vein ; w preoral pit (left head cavity). 



in which they occur, but this relation is ultimately lost, 

 when they become more closely packed and the myotomes 

 increase in size. Of these fourteen first-formed giU- 

 slits the first and the last five close up, so that eight only are 

 left. 



The atrial folds arise as ridges of the skin. Posteriorly they 

 lie close together in the middle line enclosing between them a 

 small groove (Fig. 23). Anteriorly they pass on to the right 

 side, one on either side of the gill-clefts. By the union of these 

 ridges, which begins in their posterior region and gradually 

 extends forwards, the groove becomes converted into a canal, 



* These first formed gill-slits are often called the primary gill-slits, in 

 contradistinction to the later formed secondary slits which, when the 

 ani mal becomes symmetrical, are placed on the right side, the primary 

 slits having passed over to the left side. Forster Cooper {op. cit.) describes 

 a larva taken in the open ocean in the Maldive Archipelago containing as 

 many as thirty-one primary slits. 



