230 ZOOLOGY 



enlargements occur, chief among which is the stomach or gizzard, 

 a grinding organ. In the leeches the alimentary system is 

 much modified in accordance with the blood-sucking habit of 

 the animal. The crop is capable of great enlargement and may 

 contain enough blood to nourish the animal for a long time. 

 The mouth is sometimes armed with special cuticular outgrowths 

 which serve as teeth. Glands either unicellular or compound 

 occur in" various regions of the digestive tract. These secrete 

 ferments similar to those of higher animals. In the earthworm 

 and related forms there is a dorsal longitudinal fold of the in- 

 testinal wall into the lumen of the tube, thus increasing the ex- 

 posed surface. This is called the typhlosole (Fig. 104) and is 

 supplied with cells which have been described as digestive. 

 The entodermal epithelium may secrete a cuticle or may be 

 ciliated. This layer is surrounded by connective tissue and 

 muscular fibres. 



Worms make use of every possible kind of food. Earth- 

 worms eat pieces of vegetable or animal matter, and soil con- 

 taining these. They are lovers of darkness, and capture their 

 food at night. When the light is dim they may come to the sur- 

 face of the ground and, with the tail segments inserted in the 

 opening of the burrows, explore the surroundings as far as they 

 can reach. After night showers they may come entirely out. 

 It is at such times as these that they become the prey of the 

 "early bird." Many of the marine forms are actively car- 

 nivorous. Most leeches are blood-suckers, though some are 

 carnivorous or parasitic. Many small worms flourish in foul 

 water where they use much decaying organic matter and micro- 

 scopic organisms. 



268. Respiration. The exchange of gases is effected for the 

 most part through the general body wall, into which the blood 

 capillaries or the lacunae of the coelom may penetrate. In some 

 forms there are special thin places and out-pocketings of the body- 

 wall (branchiae) by which the exchange is facilitated. These 

 are characteristic of the Polychaeta especially (Figs. 107, 108). 



269. Circulation. In some of the simplest worms there are 

 no special blood vessels. The ccelomic spaces contain a fluid, 



