6i6 



ECniNOUERMATA 



the anterior end serving as combined sense-organ and brain ; 

 a V-shaped band of cilia projecting into the oesophagus as the 

 organ of nutrition; a wide, shallow stomodaeum and an alimentary 

 canal consisting of three well-marked divisions, viz. oesophagus, 

 stomach, and intestine ; and finally a secondary body-cavity or 

 coelom, consisting of three divisions on each side, though possibly 

 the most anterior pair were confluent in the prae-oral lobe. On 

 the left side the anterior coelom opened to the exterior by a 

 short ciliated canal. To the hypothetical group so defined which 

 were certainly not Echinodermata the name 1'];otocoelomata may 

 be given. 



Now amongst the lowest types of animal in which traces of 

 Vertebrate structure can be detected, there is one group, the 



Hemichordata (Vol. VII. 



^ „' '^•f' p. 3), in which there is 



., ^ ' 'I '-^"'-^ a larva w^hich strikingly 



recalls the Dipleurula. 

 This larval form belongs 

 to Balanoglossus and is 

 called the Tornaria. It 

 possesses a well-marked 

 prae-oral lobe and a folded 

 longitudinal ciliated band, 

 which resembles that of 

 Auricularia. Its peculi- 

 arity is that in addition 

 there is a posterior ring 

 '^itiWiffliii^WiiTi^ii'T" wfirti^\ of cilia (Fig. 293, i^). 



The coelom is in five 

 divisions : — a median an- 

 ^•^ terior sac {n.c) opening 



FKi. 2%Z.— Tornaria larva. «, Amis; a.c, anterior tO the exterior by a short 

 coelom ; a.p, apical plate ; gs vadiments of ciliated Canal On the left 

 gill-sacs ; «i, mouth ; m.c, middle or " collar 



/" 



,l^.C. 



coelom ; 7), posterior ciliated hand ; p.c, posterior 

 coelom ; pr, longitudinal ciliated hand. (After 

 Morsan.) 



side ; and paired middle 

 divisions {in.c) and pos- 

 terior divisions (7>.(;). At 

 the apex of the prae-oral lobe there is a plate consisting of 

 sensory epithelium, with nerve-fibres at its base, which acts as 

 a brain. Tornaria undergoes metamorphosis, assumes a worm- 

 like form, and takes on a burrowino; life. The five divisions of 



