X] LUMBRICUS 153 



wall of the coelom, on the other hand, consists of large cubical cells, 

 the yellow cells already described. 



Beneath the visceral peritoneum there is a thin layer of circular 

 muscles, the visceral muscles derived from the peritoneal layer 

 and forming the agency by which the peristalsis (v. p. 143) of the 

 gut is carried out. 



The endoderm (8, Fig. 63) consists of a single layer of long 

 cylindrical cells bent in dorsally to form the typhlosole. Within the 

 limbs of this fold the splanchnic peritoneum is very much thickened. 



The dorsal blood-vessel can be seen embedded in the yellow 

 cells lying in the typhlosole (12, Fig. 63), whereas the ventral vessel 

 is attached by a membrane to the ventral side of the intestine. 

 This membrane is really a part of the partition which separated 

 the two coelomic sacs which originally existed in 'the segment. 



The nerve-cord, apparently lying loosely in the coelom, is sur- 

 rounded by a layer of cells similar to those forming the parietal 

 peritoneum of which they once formed a part (14, Fig. 63). Hence 

 the coelom has extended in a ring-shaped manner round the nerve- 

 cord exactly as it has surrounded the gut. At the sides of and 

 below the nerve-cord may be seen sections of vessels, the sub-neural 

 and latero-neural vessels. The mass of the nerve-cord is made up 

 of the sections of axons, whilst the nuclei of neurons can be seen 

 forming a sheath on the outer border of the cord. The fibres are 

 divided into two bundles by a septum of connective tissue. On the 

 dorsal surface of the cord there are seen three apparent tubes, which 

 are sections of the so-called "giant" fibres colossa 1 axons which 

 are outgrowths of correspondingly large neurons. 



Chaeta-sacs and nephridia cut across may be seen in some 

 sections of the worm. 



It has been mentioned above that it is one of the characteristics 

 of the coelom that the cells lining it should produce 

 the reproductive cells. This does not mean that 

 any cell lining the coelom can become an ovum or 

 a spermatozoon, but that at certain spots the cells forming part 

 of the coelomic wall turn into either female or male generative cells. 

 In the earthworm the paired ovaries (5, Fig. 64) are situated in 

 the thirteenth segment and may be seen by cutting through the 

 intestine about the region of the gizzard and gradually lifting it up 

 from behind forwards ; when it is freed up to the twelfth segment 

 the ovaries may be seen as minute white pear-shaped bodies lying 

 one on each side of the nerve-cord. They are attached by their 

 broad end to the posterior wall of the septum separating segment 



