64 ANATOMY OF AMPHIOXUS. 



bar. In this way, a complicated plexus of blood-vessels is 

 formed around and about the tubule. This vascular plexus 

 is known as a glomerulus. 



The blood charged with whatever waste matters it may 

 have gathered up in its course through the body arrives 

 eventually at the glomeruli, where it is considerably 

 delayed on account of the vascular plexus through which 

 it has to pass before reaching the dorsal aorta. During 

 this delay, it is exposed to the glandular excretory action 

 of the tubules, by which the waste products are extracted 

 from the blood by osmotic action. From the glomerulus 

 the blood is conducted by two efferent vessels, corre- 

 sponding respectively to the primary and tongue-bars, 

 into the dorsal aorta. The communication between two 

 neighbouring glomeruli, as shown in Fig. 31, is, according 

 to Boveri, the exception and not the rule. 



The distribution of these remarkable excretory tubules 

 or nephridia is coextensive with that of the pharyngeal 

 gill-clefts. They extend from the anterior to the posterior 

 extremity of the pharynx, but not beyond this. They 

 never have more than one opening into the atrial cavity, 

 but those occurring in the mid-region of the pharynx have 

 several, sometimes as many as nine, openings into the dor- 

 sal coelom. The number of ccelomic openings decreases 

 anteriorly and posteriorly, until, at the two extremities 

 of the pharynx, there is only a single coslomic opening 

 to the tubules. 



In a full-grown individual, Boveri has counted ninety- 

 one tubules on one side of the pharynx, the total number 

 therefore being double this. 



The serial distribution of the excretory tubules, one 

 after the other, is known broadly as a metamcric arrange- 

 ment. But since they correspond in number and situa- 



