244 ELEMENTARY BIOLOGY. [CHAP. 



same 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 ciliated funnel or nephrostome externally, into a vesicular 

 muscular segment, which communicates with the exterior in 

 the vicinity of the ventral pair of setae. The lining mem- 

 brane 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 metameri- 

 cally disposed series of median dorsal or peritoneal pores. 

 The precise function of these is as yet not fully understood. 



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, 

 having, so far as is known, no direct communication with 

 the body-cavity. The larger trunks of this circulatory 



