BALANO GL OSS US. 



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- 

 mertineworms. (See below.) 

 In the region of the pharynx 

 the lumen of the alimentary 

 canal is incompletely divided 

 by lateral constrictions into ^" '^ 



Fig. 118. — Transverse section through 

 two portions, an upper or the gill-region of Balanoglossus. (After 

 , , . , . . . Spengel.) 



branclnal portion carrymg ^^ Digestive portion of gut. br. 



the gill-slits, and a lower or Branchial portion of gut. bc^. Third 



^ ^ body-cavity (trunk ccclom) ; this is also 



digestive portion (Fig. 118). nearly obliterated in the adult by the pro- 



»T^i 1 ,^ 11 liferation of mesenchyme or " paren- 



The latter was compared by ^^yme" from ,ts walls. d.n.c. Dorsal 



GeGENBAUR* to the endo- nerve-cord, d.b.v. Dorsal blood-vessel. 



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

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

 it is probable that this com- Central nerve-cord. 



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

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



i.xf 



* Carl Gegenbaur, Elements of Comparative Anatomy. Translated by 

 F. Jeffrey Bell. London, 1878. 



