276 '■"^ ANIMAL KINGDOM 



the jaws open. As the pharynx is retracted the jaws close scissorswise 

 and the denticles grip the prey, dragging it back into the middle of the 

 pharynx by the time it is luUy withdrawn. 



The pharynx in Liunbricus (Fig. 15.6) is more bulbous and is at- 

 tached to the body wall by numerous radiating muscles. When these 

 muscles contract, the cavity ot the pharynx is suddenly enlarged, pro- 

 ducmg suction at the mouth. 



The pharynx leads to a tubular esophagus into which a pair of 

 glandular digestive pouches open. These pouches apparently secrete 

 digestive enzymes. In Lumbricus two pairs of calciferous glands open 

 behind the pouches. Their function is not definitely known. 



The rest of the digestive system in Nereis is a simple long intestine 

 ending at a short rectum in front of the anus. The diameter of the intes- 

 tine is smallest in the middle of each segment, and sharply expanded at 

 each septum. The moderately muscular walls have a lining of simple 

 gastrodermis, a layer of circular muscles, longitudinal muscles, and a 

 covering peritoneum. During digestion food is moved posteriorly by 

 peristaltic waves of contraction in the two muscle layers. The intestine 

 is suspended in the coelom at each septum. The mesenteries have largely 

 disappeared, remaining as bands of delicate muscle fibers dorsally and 

 ventrally in the posterior part of each segment. 



In Lumbricus the esophagus ends in an expanded storage chamber, 

 the crop. Behind the crop a muscular gizzard mills the food to a fine 

 pulp before it is passed onto the intestine where it is digested, lire 

 intestinal wall has the same layers as that of Nereis, but with much 

 thinner musculature. The intestinal diameter is greatest in the middle 

 of each segment, with moderate constrictions at each septum. The intes- 

 tine is infolded dorsally, forming externally a groove and internally a 

 ridge, the typhlosole (Fig- 15.5), which increases the absorptive surface. 

 The intestine terminates in a short rectum and anus. The peritoneum 

 surrounding the intestine in Lumbricus is modified to form a glandular 

 layer, the chloragen cells. These extract wastes from the blood, and later 

 become detached and float in the coelom. Ultimately much of their sub- 

 stance is engulfed by ameboid cells and carried to the skin where it is 

 deposited as pigment. 



As in most animals, the mouth, pharynx and rectum are lined with 

 an epidermis of ectodermal origin. In the annelids this epidermis se- 

 cretes a cuticle which is continuous with that covering the body. 



123. Nereis and Lumbricus: Circulatory System 



The annelid circulatory system is well developed. A system of large 

 vessels pumps the blood through capillary beds that invade all of the 

 tissues. The blood is collected into a longitudinal dorsal vessel (Fig. 

 15.6) and distributed from a longitudinal ventral vessel. At the anterior 

 end several pairs of commissures around the pharynx and esophagus 

 connect the two vessels. Waves of contraction force the blood forward 

 through the dorsal vessel, down the commissures, and posteriorly through 

 the ventral vessel. In Nereis the dorsal vessel is the most powerful pump, 



