284 ANIMAL BIOLOGY. [Part II. 



for certain, but it has been suggested that the function is 

 (1) excretory, to eliminate excess of carbonate of lime; (2) to 

 neutralise within the canal the humous acids produced from the 

 leaves. Absorption is mainly carried on, it may be presumed, 

 in the intestine, the sacculations and the typhlosole giving addi- 

 tional absorbent surface. 



A transverse section of the intestine shows : (1) an internal 

 membrane of columnar epithelium ; (2) a thin layer of vascular 

 connective tissue ; (3) a muscular layer, with circular fibres ; (4) 

 a longitudinal muscular layer ; (5) the so-called hepatic, or by 

 some observers, vasifactive tissue. 



The Nephridia. Excretion is effected by segmental organs or 

 nephridia, of which there is a pair for each segment, except the 

 anterior. Fig. 84, C. (after Gegenbaur and Howes) shows the 

 arrangement of the coiled tube. The internal opening is by a 

 ciliated funnel (i. o.) attached to the septum. Then follows a 

 thin-walled tube, indicated by the thin line at 1. This is suc- 

 ceeded by a thick- walled glandular ciliated portion 2, which is 

 followed by a muscular portion 3, which opens to the exterior 

 just external to the inner row of setae. The internal opening 

 is in the segment anterior to that which contains the tube, 

 which is connected to the posterior wall of the septum by a 

 mesentery not shown in the figure. The student should 

 examine in situ and mount several segmental organs, but must 

 be content if he makes out the differentiation into three por- 

 tions, and the ciliated funnel. ^ 



The Vascular System. Two fluids in the earthworm claim 

 the title of blood : (1) a colourless corpusculated fluid which 

 occupies the cavity of the segments ; (2) a red fluid, with minute 

 corpuscles, which occupies definite vessels. 



(1.) The colourless fluid resembles the blood of other inverte- 

 brates, and the lymph of vertebrates. It is found in abundance 

 in the posterior segments of the body. Its corpuscles are 

 amoebiform and nucleated. Not improbably its main function 

 is nutritive. 



