BALANOGL OSSUS. 



249 



Alimentary Canal. 



The mouth cannot be closed, as there is no sphincter 

 muscle, and accordingly, as the animal progresses through 

 the sand, it swallows a large quantity of the latter in 

 which food-particles (unicellular organisms, etc.) may also 

 be involved. As the sand passes through the intestine, 

 it becomes enveloped in the mucous secretion of the intes- 

 tinal epithelium, and is ejected through the anus in a cord 

 of slime. 



The alimentary canal is a straight tube between mouth 

 and anus. In its hinder portion it is usually sacculated, 

 i.e. provided with paired 

 lateral saccular dilatations 

 comparable to the so-called 

 intestinal cceca of the Ne- 

 mertine worms. (See below.) 

 In the region of the pharynx 

 the lumen of the alimentary 

 canal is incompletely divided 

 by lateral constrictions into , *** c 



J Fig. 118. Transverse section through 



tWO portions, an Upper Or the gill-region of Balanoglossus. (After 



, ... . . SPENGEL.) 



branchial portion carrying aL Dige sti V e portion of gut. br. 



the gill-Slits, and a lower Or Branchial portion of gut. 6c9. Third 



body-cavity (trunk ccelom) ; this is also 



digestive portion (Fig. I 1 8). nearly obliterated in the adult by the pro- 

 , , , j i liferation of mesenchyme or " paren- 



The latter was compared by chyme ., from its wal dn ^ orsal 

 GEGENBAUR * to the endo- nerve - c r d. d.b.v. Dorsal biood-vessei. 



go. Gonad. g.s. Gill- slit. t.b. Tongue- 

 Style of the AscidianS, but bar. v.b.v. Ventral blood-vessel, v.n.c. 

 , v 11 4.u iU- Ventral nerve-cord. 



it is probable that this com- 

 parison, although a very natural and useful one at the time 

 at which it was made, will not hold good, since there is 



* CARL GEGENBAUR, Elements of Comparative Anatomy. Translated by 

 F. Jeffrey Bell. London, 1878. 



