XVII 



PEOTOCHOEDATA 



597 



The tail is a most characteristic organ of the Vertebrata. It is 

 true that the name " tail " is often loosely applied to the posterior 



icon 



FIG. 435. Illustrating the mutual relationships of collar-cavities and splanchnocoeles 

 in Amphioxus lanceolatus. 



A, transverse section passing just in front of the mouth of a larva, in which theimouth and first gill- 

 slit have been formed the collar-cavities lie at the sides of the pharynx. B, transverse section through 

 the region of the first gill-slit of a larva in which the mouth has not yet been formed the splanchno- 

 coeles appear pressing the collar-cavities downwards towards the ventro-lateral angles of the pharynx ; 

 the gill-slit is seen to be formed by the meeting of ectodermal and endodermal outgrowths. C, trans- 

 verse section through the hinder end of the pharynx of a larva in which the mouth and first gill-slit 

 have been formed ; the collar-cavities have disappeared, ch, notochord ; d.gJ.ex, external opening of the 

 club-shaped gland; g.s.ed, g.s.end, ectodermal and endodermal rudiments of gill-slit respectively; /.<W/, 

 left collar-cavity ; l.spl, left splanchnocoele ; m, front lip of mouth ; MI/.I-, myocoele ; n.t, nerve tube ; 

 r.coll, right collar-cavity ; r..s-/>/, right splanchnocoele. 



part of the body of any animal, if it happens to be thin and flexible, 

 but its proper application is to a post-anal extension of the body 



