HISTOLOGY OF THE ALIMENTARY CANAL. 



93 



3. Circular Muscles (c.ni). A thin layer of muscle-fibres 

 running around the gut. 



4. Longitudinal Muscles (l.m). A thin layer of muscle- 

 fibres running along the gut. 



5. Chlw-agogue 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. This layer represents the 

 splanchnic part of the peritoneal epithelium. 



The same general arrangement exists in all parts of the alimentary 

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

 pharynx are lined by a tough, thick cuticle, and the muscular layers are 

 enormously developed. In a part of the gizzard the chloragogue-layer is 

 nearly or quite absent and the typhlosole disappears. A fuller description 

 of these modifications will be found in Brooks's Handbook of Invertebrate 

 Zoology, and a complete account in Claparede, Zeitschrift fur wissen- 

 schaftlicJie Zoologie, Vol. XIX., 1869. 



The lining epithelium is derived from the entoblast. The 

 remaining layers arise by differentiation of the splanchnic layer 

 of inesoblast. 



FIG. 40. Highly magnified cross-section through the wall of the alimentary canal, 

 eft, chloragogue layer ; c.m, circular muscles ; e.p, lining epithelium ; Z.wi, longi- 

 tudinal muscles ; v.l, vascular layer. 



Blood-vessels appear in the section as rounded or irregular 

 cavities 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 

 invested by chloragogue-cells, which radiate from them with 



