242 Elementary Biology. 



(fig. 122). Each tube is known as a nephridium (rape's, a 

 kidney), and consists of three distinct portions, a much con- 

 voluted thin portion terminated by a funnel-shaped opening, 

 a thick walled but less convoluted glandular portion, and 

 lastly a still thicker walled muscular portion which opens to 

 the exterior by a minute aperture between the outer and 

 inner rows of setae on either side and close to the inner. 

 The arrangement of the various portions of the nephridium 

 in the somite is sufficiently important to merit description. 

 Each somite, as has been already noted, is separated from 

 the neighbouring somites on either side by an incomplete 

 partition running from the body-wall inwards to the intes- 

 tine, and known as a septum or mesentery. To this septum 

 the nephridium is closely related, lying on it in a series of 

 loops, the muscuiar parts of the tube being external, and the 

 various other folds arranged on the septum in a definite 

 order. The nephridium as a whole lies. on the posterior 

 side of the septum, but the funnel-shaped inner end pierces 

 it and opens on the anterior surface, that is to say, into the 

 cavity of the somite in front of that in which the main body 

 of the nephridium lies. The cells lining the mouth of the 

 funnel are ciliated. The septum is plentifully supplied with 

 blood-vessels, many of which are intimately connected with 

 the folds of the nephridium. Doubtless the nitrogenous 

 waste is absorbed by the glandular portion of the tube, and 

 ejected by the contractile part to the exterior. 



We have now to glance at the locomotory and protect- 

 ing organs, for a skeleton or supporting system is absent 

 We have already referred to the subdivision of the body 

 into somites, separated from each other by incomplete septa. 

 We have also noted the existence of large annular spaces 

 surrounding the alimentary canal, spaces which we have 

 seen contain a (probably) nutritive fluid. It will be seen 

 that the several somitic cavities taken together really form 

 one large broken-up cavity lying between the alimentary 

 canal wall on the one hand, and the body- wall on the other ; 



