388 



HEMICHORDA 



[CH. 



which is called the glomerulus and appears to act as a kidney. 

 When the water in the cavity has become impregnated with 

 excretory products it is expelled as explained above by a muscular 

 contraction. 



The alimentary canal runs straight from the mouth on the 

 anterior surface of the collar region to the posterior end of the 

 trunk ; there is neither stomodaeum nor proctodaeum. In most 

 species in the anterior part of the trunk the canal has an 3~ sna P e 

 in section, being partially constricted into two tubes, an upper or 

 branchial into which the gill-slits open, and a lower or oesophageal 

 along which the mud is passed which the animal has swallowed for 

 food. The notochord is a hollow tube of cells surrounded by a tough 

 membrane much thickened beneath (Fig. 183). This tube opens 



FIG. 183. Longitudinal vertical section through the middle line of Glossolalanus. 



Diagrammatic. 



1. Proboscis. 2. Collar. 3. Trunk. 4. Proboscis cavity. 5. Glomerulus. 

 6. Pericardium. 7. Heart. 8. Proboscis pore. 9. Collar cavity. 

 10. Mouth. 11. Notochord. 12. Dorsal blood-vessel. 13. Oeso- 

 phageal portion of alimentary canal. 14. Branchial region of alimentary 

 canal. 15. Ventral blood-vessel. 16. Gill-slits showing external and 

 internal openings; the outlines of the external openings are dotted. 

 17. Central nervous system. 18. Dorsal roots of nervous system. 



19. Ventral pocket of proboscis cavity. 



behind into the alimentary canal in the collar region and projects 

 forward into the proboscis as a support for this organ, which is 

 attached by a very narrow neck to the collar. The whole skin is 

 sensitive, since there is everywhere a layer of nerve-fibrils under- 

 lying the ectoderm cells, but this fibrillar layer is especially thickened 

 along the mid-dorsal and mid- ventral lines of the trunk, these two 

 regions being connected by a ring of nervous tissue immediately 

 behind the collar. The dorsal thickening alone is continued into 

 the collar region, and here it becomes rolled up so as to constitute 

 a short neural tube (Fig. 183) which becomes detached from the 

 ectoderm and assumes a deeper position ; it may retain however a 



