448 The Animal Kingdom 



an outer circular and an inner longitudinal band. The longitudinal 

 muscles are bounded internally by the parietal peritoneum. 



The coelom is divided into numerous compartments by the trans- 

 verse septa, each of which extends from the body wall to the digestive 

 tract. The individual septum is not complete, for it has numerous 

 small openings through which the coelomic fluid circulates. Within 

 the coelom can be seen the paired coiled nephridia of the excretory 

 system. 



Nearly every segment possesses a pair of nephridia located laterad 

 to the digestive tract. Each nephridium is essentially a coiled tube 

 opening to the coelom through a ciliated funnel, the nephrostome, and 

 to the outside through the nephridioporc. The funnel opens in one 

 segment and the tube passes through the septa into the following 

 segments where the bulk of the coiled tube is located. The nephridi- 

 oporc is also in this latter segment. The open nephrostome contrasts 

 with the closed flame cell of the protonephridial system of the acoelo- 

 mates and some of the pseudocoelomate animals. The annelid type 

 is known as a metanephridial system. 



Like the nematodes, the annelids have a complete digestive tract. 

 While the tract is essentially a straight tube through the body, the 

 anterior portion is highly differentiated into special regions for grasp- 

 ing, storage, grinding, and digestion of food. The digestive tract is 

 provided with layers of both circular and longitudinal muscles. The 

 inner lining of the tract is formed of tall columnar, epithelial cells, 

 many of which are enlarged into unicellular glands. The outer surface 

 of the tract is covered with the visceral peritoneum. 



The circulatory system is quite varied in its details. In the majority 

 of the annelids, this is a closed system with a pulsating dorsal blood 

 vessel which pumps the blood throughout the body. In a few cases, 

 there is no special circulatory system, and the coelomic fluid assumes 

 the vascular functions. The circulatory fluid may be colorless, red 

 with hemoglobin, or, in a few cases, green due to the presence of a 

 copper compound. The hemoglobin, with few exceptions, is carried 

 in the circulating fluid and not in the corpuscles. Except for the nemer- 

 tines, the annelids are the first phylum to show a true vascular system. 

 Unquestionably the nemertines evolved this system independently. 



While a great number of the annelids simply rely upon gaseous 

 exchange through the moist body wall, some marine forms have elabor- 



