142 ORGANS AND ORGAN SYSTEMS 



Vascular Circulation The circulation of the blood is brought 

 about by peristalsis or the consecutive contraction of the circular 

 muscles in the walls of the blood vessels. This wave of con- 

 traction proceeds in the dorsal vessel from the posterior end 

 toward the anterior, the blood being forced ahead of the wave 

 as one might force water from a rubber tube. The details of 

 the path followed in the circulation of the blood are not fully 

 known, but from the dorsal vessel it passes into the numerous 

 branches of the somites, the residue going into the pharyngeal 

 blood vessel and into the aortic loops. In the ventral vessel 

 its course is from the anterior to the posterior end while in the 

 region of the stomach-intestine dorsal and ventral vessels are 

 connected by fine branching capillaries. These capillaries bring 

 waste matters from the tissues into the chlorogogue cells and 

 deposit it there to take up the digested food matters from the 

 stomach-intestine and carry them into the general circulation. 

 The exact relationship of the capillary circulation has not 

 been made out, however, and is purely conjectural at present 

 (Fig. 57)- 



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 dis- 

 sepiments in the form of slits. This fluid is made up of a color- 

 less plasm with white blood cells or leucocytes. It bathes the 

 endothelium lining the coelom and has no definite circulation 

 but is washed back and forth by movements of the worm. 

 / F. THE EXCRETORY SYSTEM. Nephridia. The waste matters 

 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 ejaculatory 

 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 



