PHYLUM ANNELIDA. SEGMENTED WORMS 



ii: 



from the dorsal vessel supplies the dorsal 

 half of the intestine. 



The dorsal vessel (Fig. 93) serves the 

 function of a true heart in that it is a pump 

 with valves; and the aortic arches, the so- 

 called hearts, act as a pressure-regulating 

 mechanism, receiving blood in spurts from 

 the dorsal vessel, and then contracting to 

 force the blood under a steady pressure into 

 the ventral vessel. Blood is forced forward 

 by wavelike contractions of the dorsal ves- 

 sel, beginning at the posterior end and 

 traveling quickly anteriorly. These contrac- 

 tions are said to be peristaltic; they have 

 been likened to the action of the fingers in 

 the operation of milking a cow. Valves 

 (Fig. 93) in the walls of the dorsal vessel 

 prevent the return of blood from the an- 

 terior end. In segments 7 to 11, the blood 

 passes from the dorsal vessel into the hearts, 

 which force it forward and backward in the 

 ventral trunk. Valves in the heart prevent 

 the backward flow. From the ventral vessel 

 the blood passes to the body wall, the in- 

 testine, and the nephridia. The flow in the 

 subneural vessel is toward the posterior end, 

 then dorsally through the parietal vessels 

 into the dorsal vessel. The anterior region 



receives blood from the dorsal and ventral 

 vessels. The blood which is carried to the 

 body wall and the skin receives oxygen 

 through the cuticle and is then returned to 

 the dorsal vessel by way of the subneural 

 vessel and the parietal connectives. 



The exchange of materials between the 

 blood and the tissue cells takes place in 

 minute tissue spaces. Blood plasma and a 

 few corpuscles, which constitute the tissue 

 fluid, pass from the capillaries into these 

 tissue spaces, where the cells are bathed 

 and the interchange occurs. The tissue fluid 

 collects waste products of cellular metab- 

 olism and makes its way back again into the 

 blood stream. 



Respiration 



The earthworm possesses no organized 

 respiratory system, but it obtains oxygen and 

 gets rid of carbon dioxide through the moist 

 skin. Respiration can be carried on in air 

 and also in water as experiments have 

 shown. Many capillaries lie just beneath the 

 cuticle, making transfer of gases essentially 

 as it is done in a gill or lung. The oxygen 

 passes into the blood and combines with the 

 hemoglobin. The hemoglobin of the earth- 



Cerebral ganglia "brain" Pharynx Lateral nerve 



Tactile nerve 

 Prostomium 



OJI 



Mouth 



Mouth 

 cavity 



Circumpharyngeal 

 connective 



Subpharyngeal 

 ganglion 



Nerve 

 cord 



Figure 94. Earthworm. Side view of anterior end showing the cerebral gangha and large; 

 nerves. (After Hess.) 



