2. Yellow granules, from the cholorogogue cells (See D II. i). 



3. Bacteria or other foreign bodies, especially Gregarina, which 

 are often present. 



D. HISTOLOGY. 



I. BODY \YALL. 



Examine prepared transverse sections of the body ; observe body 

 wall, now seen to be made up of five layers : 



1. Cuticle, a thin non-cellular layer (membrane) often torn off. 



2. Deric epithelium or epidermis, a single layer of cells many 

 of which are swollen (gland cells). 



3. A thin outer layer of circular muscle fibres with blood vessels 

 and connective tissue nuclei among them. 



4. A thick layer of longitudinal muscle fibres or plates, arranged 

 in elongated groups of eliptical form. 



5. Peritoneum or coelomic epithelium, a thin layer of granular 

 protoplasm containing nuclei, lining the body cavity. 



Sketch a portion of the body wall to show the above parts. 



II. DIGESTIVE TRACT. 



In its wall four layers are to be seen. 



1. Chlorogogue cells, large and more or less elongated and irreg- 

 ular. 



2. An outer layer of longitudinal muscle fibres cut across. 



3. A layer of circular muscle fibres and of blood vessels (not 

 easily made out ) . 



4. Enteric epithelium ; a single layer of elongated cells with 

 stained nuclei and a thin cuticle over their central ends through 

 which fine cilia project into the lumen of the gut. 



Sketch a small part of the digestive tract to show the above. 



III. NERVOUS SYSTEM. 



In a transverse section of the ventral cord, note : 



1. An outer muscular sheath or coat. 



2. Large ganglion cells in groups or clusters. 



3. A mesh work of fine fibres. 



4. Yery large clear "giant fibres" each in a definite sheath. 



5. In some of the sections the nerves are to be seen as they pass 

 from ganglion cells to the body wall. 



IY. BODY CAVITY. 



Some of the sections will show the following structures : 

 i. The corpuscles of the coelomic fluid. 



44 



