XYII 



PEOTOCHOEDATA 



597 



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

 true that the name « tail " is often loosely apphed to the posterior 



icoll /• 



end 



Fig. 435.- 



-lUustrating the mutual relationships of collar-cavities and splanchnocoeles 

 in Amphioxus lanceolatiis. 



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

 islit 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-Iateral 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 ; cl.gl.ex, external openingof the 

 club-shaped gland ; g.s.eet, g.s.end, ectodermal and endodermal rudiments of gill-slit respectively ; l.colly 

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

 r.coU, right collar-cavity ; r.spl, 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 



