HIGH SCHOOL ZOOLOGY. 



219 



glandular in their character, and thus the skin is softer than in 

 the Arthropods. The external segmentation does not always 

 correspond to the internal ; in the earthworm it does, partitions 

 or septa being attached opposite the external furrows, which 

 tend to divide off the coilom into so many chambers as there are 

 segments. In the leech, however, there are sevei-al furrows, 

 which are merely skin-deep, to each true segment marked off 

 by the septa. The completeness of the internal segmentation, 

 bi'ought about by these septa, necessarily affects the various 

 organs contained in the coelom, thus the intestine, the blood- 

 vessels and. the nerve-cord all partake in it. It is not often 

 that we observe any reduction in the number of repeated parts, 

 (each segment, for example, having its own nerve-ganglion), and 

 this is especially true of the excretory system, each segment 

 having a pair of coiled tubes, segmental organs or nephridia, 

 which open outwardly and also into the coelom (Fig. 148). Ho- 



S' 



Fig. 148.— Diagram of transection of earthworm. 



IT, the hjTJodermis ; c, the circular, I, the longitudinal muscular layers ; S< S-, 

 the upper and lower jxiirs of bristles; a, external, a', c(oloniic aperture of tlie 

 iiephridium (the external aperture is not exactly in the same plane as the setLC, nor is 

 the internal aperture in the same plane as the external) ; i, the intestine with Its roof 

 infolded (typhlosole) : it is coated with glandular epithelial cells; blood-vessels are 

 represented in black, above and below the intestine, and around the nerve-cord— w. 



