6i6 



ECHINODERMATA 



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 Peotocoelomata 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. 



op 



vs 



pr 



ni.C. 



l^.C. 



p. 3), in which there is 

 a larva which 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 

 of cilia (Fig. 293, p). 

 The coelom is in five 

 divisions : — a median an- 

 ^ terior sac {a.c) opening 



a, Anus ; a.c, anterior to the exterior by a short 



coelom ; a.p, apical plate ; g^s radiments of filiated canal on the left 

 giU-sacs ; ?n, montli ; m.c, middle or " collar 



coelom ; J), posterior ciliated band ; j5. c, posterior side; and paired middle 



coelom ; pr, longitudinal ciliated baud. (After j • • • / \ j 



Morgan!) divisions {m.c) and pos- 



terior divisions {p-c). 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 burrowing life. The five divisions of 



Fig. 293. — Toiiw.ria. larva. 



