PHYLUM ANNELroA. SEGMENTED WORMS 



175 



eter of the body becomes smaller and the 

 worm longer. Under the circular layer is a 

 thick longitudinal layer with muscle fibers 

 lying parallel to the length of the worm; 

 when these contract, the diameter of the 

 body becomes greater and the worm shorter. 



Digestive system 



The digestive tract (Fig. 91) consists of 

 (1) a mouth (buccal) cavity in segments 

 1 to 3; (2) a thick muscular pharynx lying 

 in segments 4 and 5; (3) a narrow, straight 

 tube, the esophagus, which extends through 

 segments 6 to 14; (4) a thin-walled enlarge- 

 ment, the crop ( proventriculus ) , in seg- 

 ments 15 and 16; (5) a thick muscular- 

 walled gizzard in segments 17 and 18; and 

 (6) a thin-walled intestine extending from 

 segment 19 to the anal opening. The intes- 

 tine is not a simple cylindrical tube, its 

 dorsal wall is infolded, forming an internal 

 longitudinal ridge, the typhlosole ( Fig. 92 ) ; 

 this increases the digestive surface. Sur- 

 rounding the digestive tract and dorsal blood 

 vessel is a layer of chloragogue cells (Fig. 

 92). The functions of these cells are not 

 known with certainty, but in lumbricids and 

 some other groups they nourish the develop- 

 ing eggs. Since chloragogue cells can synthe- 

 size urea, they are thought to be also ex- 

 cretory. Three pairs of calciferous glands 

 lie at the sides of the esophagus in segments 

 10 to 12; actually, the first pair are storage 

 pouches, but the second and third are true 

 glands. Their primary function is excretion 

 of calcium; neutralization of acid foods is 

 probably an incidental function. 



The food of the earthworm consists prin- 

 cipally of pieces of leaves and other vegeta- 

 tion, particles of animal matter, and soil; 

 this material is gathered at night, at which 

 time the worms are active. They crawl out 

 on the surface of the ground and hold fast 

 to the tops of their burrows with their tails, 

 exploring the neighborhood. Food parti- 

 cles are drawn into the mouth cavity by 

 suction produced when the pharyngeal 

 cavity is enlarged by the contraction of the 



muscles which extend from the pharynx 

 to the body wall. 



In the pharynx, the food receives a secre- 

 tion from the phar}'ngeal glands; it then 

 passes through the esophagus to the crop, 

 where it is stored temporarily. The gizzard 

 is a grinding organ; in it the food is broken 

 up into minute fragments by being squeezed 

 and rolled about. Solid particles, such as 

 grains of sand, which are frequently swal- 

 lowed, probably aid in this grinding proc- 

 ess. The food then passes on to the intes- 

 tine, where most of the digestion and 

 absorption takes place. 



Digestion in the earthworm is very simi- 

 lar to that of higher animals. Enzymes aid 

 in the breakdown of food; these include 

 amylase which acts upon carbohydrates, 

 cellulase which acts upon cellulose, pepsin 

 and trypsin which act upon proteins, and 

 lipase which acts upon fats. The digested 

 food is absorbed through the wall of the 

 intestine, assisted by the amoeboid activity 

 of some of the epithelial cells. Upon reach- 

 ing the blood, the absorbed food is carried 

 to various parts of the body. Absorbed food 

 also makes its way into the coelomic cavity 

 and is carried directly to those tissues bathed 

 by the coelomic fluid. In one-celled animals, 

 and in such metazoans as the hydra, planaria, 

 and ascaris, no circulatory system is neces- 

 sary, since the food is either digested within 

 the cells or comes into direct contact with 

 them; but in large complex animals, a spe- 

 cial system of organs must be provided to 

 bring about the proper distribution of di- 

 gested food. 



Circulatory system 



The blood of the earthworm is contained 

 in a complicated system of tubes which 

 ramify to all parts of the body (Figs. 91 

 and 93). A number of these tubes are large 

 and centrally located; these give off branches 

 which likewise branch, finally ending in ex- 

 ceedingly thin tubules, the capillaries. The 

 blood consists of a plasma in which are 

 suspended a great number of colorless 



