244 ELEMENTARY BIOLOGY. [CHAP. 



for each except that that which is external in the one wall 

 is internal in the other and vice versa. 



The body-cavity is subdivided by the mesenteric septa 

 into a series of somitic compartments, and the metameric 

 symmetry thus established extends to the excretory organs, 

 there being one pair of these attached to each septum, the 

 first three somites excepted. These organs, being thus 

 segmentally arranged, are termed segmental organs or 

 nephridia. Each consists of a tortuous tube which can be 

 resolved into three segments; a middle glandular one, 

 abundantly supplied with blood-vessels, passing — internally, 

 into a delicate thin-walled loop, which perforates the 

 mesentery and opens into the segment in front by means of 

 a complex ciliated funnel or nephrostome — externally, into a 

 vesicular muscular segment, which communicates with the 

 exterior in the vicinity of the ventral pair of sets. The 

 lining membrane of these excretory tubules is profusely 

 ciliated, and a current is thus induced from within outwardly. 

 In addition to the indirect communication established, 

 through the agency of the nephridia, between the body- 

 cavity and the exterior, a direct one is instituted by means 

 of a metamerically disposed series of median dorsal or 

 peritoneal pores. There is reason to believe that the milky 

 fluid ejected through these is functional in facilitating the 

 movements of the animal within its burrow. 



The mesenteric septa themselves are incomplete ventrally; 

 they do not subdivide the body-cavity into a series of closed 

 chambers. As this is so, it follows that that cavity is a con- 

 tinuous one in open communication with the surrounding 

 medium. It contains during life a colourless perivisceral 

 fluid, in which there are present immense numbers of 

 nucleated amoeboid corpuscles. 



The true red-blood fluid circulates in a system of vessels, 



