292 THE EARTHWORM AND OTHER ANNULATA 



fibers, connective tissue, and small blood vessels, and which 

 separate at the body wall like the two layers of peritoneum that 

 form the mesenteries of a vertebrate animal (Fig. 19, p. 40). 



Digestive System. — The mouth leads into a short buccal region 

 (Figs. 137 C and 139). This is followed by the pharynx, which is 

 the muscular organ concerned in the sucking action of the lips. 

 There are numerous radial muscles extending outward from the 

 walls of the pharynx to the inner surface of the body wall, and 

 within its wall there are well-developed circular muscles. When the 

 mouth is applied to an object the contraction of the radial muscles 

 expands the cavity of the pharynx, so that the lips hold fast, like 

 the edges of a vacuum cup. The circular muscles restore the 

 pharynx again to its original dimensions. The effectiveness of 

 this action may be judged from the way in which the worms can 

 drag relatively large objects, like leaves, toward their burrows 

 and " nibble away " the surfaces that have been softened by the 

 saliva. The esophagus is a comparatively slender region of the 

 tract that leads from the pharynx. On the walls of the esophagus, 

 in segments eleven and twelve, are the paired calciferous glands, 

 the secretion of which passes into the esophagus in segment eleven. 

 The function of this secretion is somewhat problematical, although 

 it has been supposed that the carbonate of lime neutralizes the 

 free acid, which may be contained in the food, and thus enables 

 the intestinal enzymes to function more effectively. Posterior to the 

 esophagus in segments thirteen, fourteen, and fifteen is the crop, 

 which, as its name implies, serves as a place of temporary storage, 

 anterior to the muscular gizzard, where the food is reduced to a 

 more finely divided state. The sand grains and minute pebbles 

 that are swallowed with the food seem to be used in the gizzard as 

 in the part of the digestive tract which is given the same name in 

 birds. After being thus reduced to a finer condition and thor- 

 oughly mixed, the food passes through the valve-like aperture at 

 the posterior end of the gizzard into the stomach-intestine, which 

 is a thin-walled tube that is capable of much dilation between 

 the septa when distended with food. A longitudinal fold, the 

 typhlosole, which protrudes from its dorsal wall (Fig. 141) and 

 which increases the surface area available for digestion and absorp- 

 tion, may be compared with the foldings observed in the mucous 

 membrane of the stomach and intestine of a vertebrate animal 

 (c/. Fig. 83, p. 150). The faeces of the earthworm consist 



