EARTHWORM 351 



suspended in the plasma and not in the corpuscles as is the case in 

 many animals. A complicated system of blood vessels makes up the 

 circulatory path of the blood. The principal ones are: (1) the 

 dorsal blood vessel, (2) the paired hearts (usually five) in segments 

 VII to XI, (3) the ventral blood vessel, (4) the subneural trunk, (5) 

 the parietal vessels, (6) the typhlosolar vessel, and (7) the intestino- 

 integumentary vessels. The dorsal vessel conveys the blood an- 

 teriorly and forces it along by wavelike contractions. The paired 

 hearts receive the blood from the dorsal vessel and by pulsating move- 

 ments force it into the ventral vessel which distributes it to the body 

 wall, the nephridia, and the intestine. In the intestine food is taken 

 up ; in the skin gaseous exchanges are made with the water in the 

 moist soil ; and in the nephridia the nitrogenous wastes are removed. 

 The lateral neural vessels receive freshly oxygenated blood from the 

 skin; hence the nervous system receives the most highly oxygenated 

 blood. From the lateral vessels it passes into the subneural, where 

 it flows posteriorly, and then returns to the dorsal vessel by way of 

 the parietal connectives. The blood flows from the intestine through 

 the typhlosole into the dorsal vessel by dorso-intestinal vessels. An- 

 terior to the hearts the dorsal vessel carries the blood posteriorly 

 and the ventral vessel carries it anteriorly. The circulatory system 

 is equipped with numerous valves which keep the blood from flow- 

 ing in the wrong direction. 



Respiratory System 



Respiration in the earthworm is carried on through the skin which 

 is well supplied with blood. Since the animal always lives in a 

 moist environment, this type of respiration is possible. 



Excretory System 



The function of excretion is cared for principally by the paired 

 nephridia, which are found in each segment except the first two and 

 the last one. A single nephridium consists of a ciliated funnel (the 

 nephrostome) , a thin coiled tube, and a nephridiopore. The cilia of 

 the nephrostome create a current which takes the fluid containing 

 nitrogenous wastes from the coelom into the tubule where it can pass 

 to the outside through the nephridiopore. Also the wastes in the 

 blood are excreted by way of the nephridial tubules. The nephro- 



