5&6 INVERTEBEATA 



the collar -cavities, as seen from the side, is therefore thai 

 reversed S- 



The next changes which occur are, the enlargement of tht 

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

 the mouth. The mouth appears on the lelt side of the anima 

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

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

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

 ectoderm and acquires an opening to the exterior ; whilst th( 

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

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

 as Hatschek's pit. 



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

 of a downgrowth of the pharynx with an ectodermal ing 



Simultaneously a str 

 PP ^f called the club-shaped 



' ' makes its appearance, 

 arises as a pocket, hi 

 on the right wall c 

 \£ pharynx opposite the i 

 This pocket curves 

 ward and forward, and 

 to the exterior below •< 

 ]■ ^ I front of the mouth. 



'^'^^ the openmg of the 



Pig. 43-4.— Diagram of longitudinal sagittal sec- shaped gland the waU 



tion of Amphioxus lanceolatus to one side of „>,„„„„„ lippninpa rr., 



the middle line, iu order to show the mutual P^iaiynx uecomes m( 



relationships of head, collar, and trunk cavities. tO form a V-Shapcd 



col.coe., collar coelomic pouch ; h.coc, head coelomic witll the apCX 01 1 



pouch; n.c, neurenteric canal; n.f, neural fold; n.p, llolntlTlO' in waTfls Th( 



neural plate ; tr.coe, trunk coelomic pouch. 1^ . ^ ,. ^ * - - 



consists 01 glandular 

 and of cells carrying short cilia, and it is the rudiment ( 

 endostyle of the adult (endst, Figs. 436, 437, 439, 441). 



This extraordinary larva, which now possesses mouth and 

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

 three months. During this period new somites are added b 

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

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

 of the larva. The development of these has been worked ( 

 Lankester and Willey (1890), who term them primary gil. 

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

 primary gill-slits are formed. 



The hinder ends of the coelomic grooves or trunk -cavities 

 by this time become soUd and have finally separated from th( 

 the neurenteric canal also has become a solid string of cells, an( 

 and the neighbouring tissues, give rise to a growing zone, thi 

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

 cord, notochord, and myotomes, known as the tail (Fig. 43*7). 



