THE ANATOMY OF SUALIBREGMA INFLATUM. 281 



The nephridium may, for purposes of description, be divided 

 into four regions : (1) tlie funnel, (2) the fine straight tube 

 wliicli leads directly backwards from the funnel into (3) tbe 

 loop formed by a U-sliaped tube, the two limbs of which are 

 parallel and close together, the' second limb leading into (4) 

 the short termiual tube of the nephridium (fig. 18). The only 

 part of the nephridium visible without dissection is the loop, 

 which is generally seen through the intervals between the 

 oblique muscle bands. 



The funnel is very small; even in large worms it is seldom 

 more than "4 mm. in diameter. The opening is generally 

 directed ventrall}'. Both lips are simple, and bear no pro- 

 cesses. The dorsal lip is larger than the ventral one, and 

 overhangs the aperture like a hood. Occasionally in large 

 nephridia the dorsal lip is slightly folded. The funnel is 

 difficult to see because it is partially hidden by several 

 blindly ending processes of the afferent nephridial vessel, 

 which are bound to the funnel by a strand of connective 

 tissue which represents the septum in the post-diaphragmatic 

 segments (see p. 255). The funnels of the first three 

 nephridia are situated on the anterior faces of the second, 

 third, and fourth diaphragms respectively. 



The funnel leads into a short ciliated tube, which generally 

 runs directly backwards. This portion, even in the largest 

 nephridia, is only about 1 mm. long, and its lumen is small 

 (about "04 mm. in diameter). 



The two limbs of the loop of the nephridium, which form 

 the excretory part of the organ, are closely applied together. 

 Each is ciliated. The lumen of the straight tube of the 

 nephridium undergoes a gradual enlargement as this tube 

 merges into the first limb of the loop, the lumen of the latter 

 portion being about '08 mm. wide in large specimens. A 

 little more than halfway along the first limb of the loop 

 there is a distinct narrowing of the lumen, which throughout 

 the second limb is only about '04 mm. in diameter, so that 

 the two limbs of the loop may be easily distinguished in 

 section by the relative sizes of their cavities (see fig. 16). 



