158 BIOLOGY 



The Alimentary Canal. The alimentary canal (enteron) is 

 a straight tube extending from one end of the animal to the 

 other, without any convolutions. It does, however, show 

 several distinct regions. The mouth opens into a slightly 

 swollen section known as the throat or pharynx, ph. The 

 pharyngeal walls are muscular, with a radiating series of 

 muscles that pass outward to be attached to the body wall, 

 mu. The contraction of these muscles will cause an expansion 

 of the pharynx and convert it into a sucking organ by means 

 of which the animal draws food into its mouth. Behind the 

 pharynx the canal contracts into a straight gullet or oesopha- 

 gus, oe, which continues back to the fifteenth segment. Here 

 it enlarges into a thin-walled crop, cr, which is followed in 

 the fifteenth and seventeenth segments by a second enlarge- 

 ment with thicker walls, called the gizzard, g. Beyond this 

 the intestine extends in a straight line to the anal aperture 

 or vent. The intestine is not a simple cylindrical tube but has 

 its dorsal side folded inward to form a longitudinal ridge known 

 as the typhlosole (Gr. typhlos = blind + solen = tube), ty (Fig. 

 81), whose purpose seems to be only to increase the amount 

 of interior surface within the intestine. 



Circulatory System. The circulatory system consists of two 

 parts, the blood system and the codomic fluid. 



The blood system. A series of tubes or vessels containing 

 blood comprises the circulatory system. The blood of the 

 earthworm is red, a very unusual condition among lower ani- 

 mals. The red color is due to a substance called haemoglobin 

 (Gr. haima = blood + Lat. globus = globe), which is dissolved 

 in the liquid part of the blood, and is not contained in the 

 corpuscles, as it is in the frog and higher animals. This blood 

 is kept in constant motion in the vessels, forced along by their 

 contractions. The chief vessels and the direction of the blood 

 current are shown in Figure 77 and they are as follows: 



Running anteroposteriorly, just above the alimentary tract, 

 is a large longitudinal dorsal vessel, dv, with muscular walls. 



