144 ^N INTRODUCTION TO THE STUDY OF FOSSILS 



strong, notched, black chitinous jaws. After seizing its prey with 

 these jaws, the proboscis is rolled in again and the jaws then tear 

 the food into pieces. The food next passes through the narrow 

 oesophagus, into the long, straight intestine ; the anus is at 

 the posterior end of the tail segment. The intestine is regularly 

 narrowed, corresponding to the surface constrictions. Into the 

 oesophagus, from the glands at its sides, is poured a fluid, prob- 

 ably digestive in function. From within outward the walls 

 of the intestine consist of (i) a layer of epithelial cells, (2) a 

 layer of circular muscles, (3) a layer of longitudinal muscles 

 and (4) a surface membrane. The muscles aid the forward 

 movement of the food, while the innermost layer probably aids 

 in absorbing the digested portions of the food. Since both the 

 anterior portion of the digestive canal to the beginning of the 

 oesophagus, and the posterior portion within the tail segment 

 are developmentally invaginations of the external surface they 

 are similarly lined with chitin. 



The blood, of a bright red color, is carried in two main blood 

 vessels, a dorsal tube carrying it forward and a ventral tube 

 moving it backward ; connecting these two are loops in each 

 segment and from these loops branches are given off to the diges- 

 tive canal and to the parapodia. There is no distinct heart, but 

 in the walls of most of the blood vessels are rings of muscles 

 which, contracting in succession, move the blood forward; the 

 walls themselves are likewise contractile. These wave-like 

 contractions move the blood from behind forwards in the dorsal 

 vessel and from before backwards in the ventral vessel. 



Respiration takes place through the entire body surface but 

 especially through the leaf-shaped gill-lobe attached to the upper 

 side of each parapodium ; these lobes are green at the head end 

 of the body but bright red at the middle and posterior parts. 



While the excretion of CO2, etc., takes place largely through 

 the gill-lobes of the parapodia, the nitrogenous waste (uric acid, 

 etc.) is carried off by a pair of curved tubes {nephridia) in each 

 segment. 



