146 ORGANS AND ORGAN SYSTEMS 



has passed, while blood is prevented from flowing back into the 

 parietal vessel by closure of the valve, due to pressure of the 

 oncoming peristaltic wave. 



The dorsal vessel thus functions like a heart, the force neces- 

 sary to propel the blood through the large vessels and capillaries 

 coming from the consecutive contractions, or peristalsis, of its 

 circular muscles, and the force, in turn, comes from the trans- 

 formed energy, due to oxidation, in the muscle cells. 



Coelomic Circulation. Another circulating fluid is contained 

 in the body cavity, or coelom, which is continuous throughout 

 all of the somites of the worm through dorsal apertures, in the 

 form of slits, between the dissepiments and the digestive tract. 

 This fluid is made up of a colorless plasm with white blood cells 

 or leucocytes. It is washed back and forth by movements of 

 the worm, thus bathing the endothelium lining the coelom, but 

 there is no definite circulation. 



F. THE EXCRETORY SYSTEM. Nephridia- -The waste mat- 

 ters of metabolism are disposed of through the action of small 

 but complicated organs, called nephridia, a pair of which may be 

 found in all of the somites after the first four. Each nephridium 

 consists of similar parts, the most important of which are: (i) 

 the funnel or nephrostome, (2) the ciliated neck, (3) the coiled 

 narrow tube, (4) the wide glandular tube, and (5) the ejacula- 

 tory duct opening to the outside (Fig. 58). 



The ciliated neck of the nephrostome passes through the 

 anterior wall of the somite, close to the mid-ventral line. The 

 nephrostome, therefore, lies in the somite anterior to the one 

 containing its own nephridium, so that waste matters of any one 

 somite are expelled to the outside by the nephridium of the next 

 posterior somite. The nephrostomes or mouths of the nephridia 

 are flattened fan-like structures, consisting of two flattened 

 lamellae or plates with a narrow slit-like opening between them ; 

 the great cells lining the opening are covered with powerful cilia 

 which maintain a constant current toward the tubular part of 

 the nephridium. These tubes are developed in coils which lie 

 in the posterior parts of the somites, three coils or turns in each, 

 the third ending in an enlarged portion opening to the outside 



