158 



THE BIOLOGY OF AN ANIMAL. 



4. Circular Muscles (c.m}. A thin layer of muscle-fibres run- 

 ning around the gut. 



5. Longitudinal Muscles (l.iri). A thin layer of muscle-fibres 

 running along the gut. 



6. Chloragogue Layer (cK). Composed of large polyhedral 

 or rounded cells containing yellowish-green granules. The cells 

 fill the hollow of the typhlosole, and cover the surface of the 

 dorsal and lateral blood-vessels. 



7. Peritoneal Epithelium (not shown in the figure). 



The same general arrangement exists in all parts of the alimentary 

 canal, but is sometimes greatly modified. For instance, in the gizzard and 

 pharynx the muscular layers are enormously developed and the cuticle is 

 thick. In a part of the gizzard the chloragogue-layer is nearly or quite ab- 

 sent and the typhlosole disappears. A fuller description of these modifi- 

 cations will be found in Brooks's Handbook of Invertebrate Zoology, and 

 a complete account in Claparede, Zeitschrift filr luissenschaftliche Zoo- 

 logie, Vol. XIX., 1869. 



The lining epithelium and the cuticle are derived from the 

 entoblast. The remaining layers arise by differentiation of the 

 splanchnic layer of mesoblast. 



: > : 



-> ..: -.;:>.: '^:;>-.' 



FIG. 88. Highly magnified cross-section through the wall of the alimentary canal, c, 

 cuticle ou the inside; ch, chloragogue layer; c.m, circular muscles; ep, lining epithe- 

 lium; l.m, longitudinal muscles; v.l, vascular layer. 



Blood-vessels appear in the section as rounded or irregular cav- 

 ities bounded by thin walls. They consist of a delicate lining 

 epithelium covered by a thin layer of muscle-fibres. In the 

 walls of the stomach-intestine the vessels are often completely 



