196 



INVERTEBEATA 



CHAP. 





the collar -cavities, as seen from the side, is therefore that of a 

 reversed S- 



The next changes which occur are, the enlargement of the front 

 end of the gut as the rudiment of a pharynx, and the formation of 

 the mouth. The mouth appears on the left side of the animal; the 

 pharyngeal wall adheres at this spot to the ectoderm and a perforation 

 takes place (m, Fig. 435). Soon after this the left head-cavity, which 

 is small and spherical and lined with columnar cells, fuses with the 

 ectoderm and acquires an opening to the exterior ; whilst the right 

 one becomes thin-walled, and extends to the extreme anterior end of 

 the larva beneath the notochord (Fig. 433, B). It is known henceforth 

 as Hatschek's pit. 



The first gill-slit is formed in the mid-ventral line by the union 

 of a dbwngrowth of the pharynx with an ectoderrnal ingrowth. 



Simultaneously a structure 

 n f called the club-shaped gland 

 makes its appearance. This 

 arises as a pocket, high up 

 on the right wall of the 

 pharynx opposite the mouth. 

 This pocket curves down- 

 ward and forward, and opens 

 to the exterior below and in 

 front of the mouth. Below 

 the opening of the club- 

 shaped gland the wall of the 

 pharynx becomes modified 

 to form a V-shaped ridge, 

 with the apex of the V 

 pointing inwards. The ridge 

 consists of glandular cells 

 it is the rudiment of the 



col.coe 



trcoe 



FIG. 434. Diagram of longitudinal sagittal sec- 

 tion of Amphioxus lanccolatus to one side of 

 the middle line, in order to show the mutual 

 relationships of head, collar, and trunk cavities. 



col.coe, collar coelomic pouch ; It.. me, head coelomic 

 pouch ; n.e, neurenteric canal ; ./, neural fold ; ./>, 

 neural plate ; tr.coe, trunk coelomic pouch. 



and of cells carrying short cilia, and 



endostyle of the adult (en<lst, Figs. 436, 437, 439, 441). 



This extraordinary larva, which now possesses mouth and anus, 

 and is able to feed itself, leads an active swimming life for about 

 three months. During this period new somites are added behind, 

 and new gill-slits appear in a single series behind the first ; but they 

 are no longer mid-ventral, but are actually situated on the right side 

 of the larva. The development of these has been worked out by 

 Lankester and Willey (1890), who term them primary gill-slits. 

 As many as fourteen (according to Goodrich [1910] fifteen) of these 

 primary gill-slits are formed. 



The hinder ends of the coelomic grooves or trunk-cavities have 

 by this time become solid and have finally separated from the gut ; 

 the neurenteric canal also has become a solid string of cells, and this, 

 and the neighbouring tissues, give rise to a growing zone, the tail- 

 bud. From this bud there grows out a post-anal extension of nerve 

 cord, notochord, and myotomes, known as the tail (Fig. 437). 



