III.] THE EARTHWORM. 265 



c. The mesenteric septum. A thin fibrous sheet pass- 

 ing from a to b. Note its circular and radial 

 musmlar fibres. 



It is perforated ventrally {circum-tieural arcade) 

 to give passage to the nervous axis and its four 

 accompanying blood-vessels. 



Look for the nephridia — portions of them will 

 certainly be visible in the thickest sections. Note 

 especially their communication with the exterior, 

 each by a narrow duct passing externally to the 

 ventral setae. 



Look for the dorsal pore^ for sections of the great 

 blood-vessels and traces of their branches (see 

 Sect. F). 



2. Examine a thin portion of the body-wall of the above, 

 under a high power. Note — 



a. The epider7nis ; composed of a layer of elongated 

 cells. They stain very lightly, and their nuclei 

 are very small. 



b. The unicellular cutaneous glands^ large ovoidal 

 structures buried up in a. Each opens on to 

 the exterior by a short neck. They are rendered 

 very conspicuous by the affinity of their granular 

 contents for the staining reagent. 



c. The cuticular exoskeleton, a delicate investment for 

 a. It is structureless, and stains uniformly and 

 with moderate intensity. 



It is frequently puckered or otherwise displaced 

 in the process of manipulation. 



d. TYiQ circular muscular layer. It lies immediately 

 beneath, and is two or three times the transverse 

 diameter of, a. 



