PEOTOCHOEDATA 



597 



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

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



Icoll 



rcoll 



cl.gl.ex 



gs.end 



n.t 



FIG. 43fi. Illustrating the mutual relationships of collar-cavities anil splanchnocoeles 



in Amphio.i'iis lanceolatits. 



A, transverse section ] Kissing, just in front of tin- mouth of;t larva, in which thei ulli and lirst ^ill- 

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

 the region of tin- lirst ^ill-slit of a larva in which the month has not. yet. tieen formed the .splanchno- 

 coeles appear piessing the collar-cavities down wan Is towards the ventro-lateral angles of the pharynx ; 

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

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

 have been formed ; the collar-cavities have disappeared, c/i, notochon! ; cl.gl.ex, external opening of the 

 club-shaped gland; <M-<'''', <j.x.cwl, ectodermal and endodermal rudiments of gill-slil resjii'ci ivel.v ; l.coll, 

 left collar-cavity ; l.*i'l, left. s])lanchnocoele ; m, front lip of month ; my.c, niyocoele ; //./, uei \e I ube . 

 /.CM//, i JLjht collar-cavity ; r.x t il, riglit s]]]anchnocoele. 



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 



