CHAPTER XXII 

 THE ARTHROPODA APUS CANCRIFORMIS 



WE have seen that the Earthworm, a representative of the 

 class Annelida chcetopoda, has a segmented body following 

 upon a minute pre-oral region called the prostomium. Each 

 body-segment or somite is represented externally by an annulus, 

 and bears, in nearly every case, four couples of locomotory 

 bristles. Corresponding to the external annulations we found an 

 internal segmentation, most perfectly expressed by the division 

 of the body-cavity into a number of chambers separated by 

 muscular partitions called septa. In every such chamber we 

 found a pair of coiled and ciliated excretory tubules, the 

 nephridia, opening into the body-cavity of one segment by 

 ciliated funnels, and to the exterior in the segment next behind 

 by excretory pores. These nephridia were shown to have been 

 formed from peculiar rows of cells forming part of the ectoderm 

 of the embryo, and to have grown from the ectoderm towards 

 the body-cavity, acquiring an opening into the latter at a 

 comparatively late stage of development. Further, it has been 

 shown that the generative ducts, so similar in their general 

 characters to the nephridia, have a different developmental 

 history, being formed as outgrowths from the body-cavity 

 towards the ectoderm, which they met, and eventually fused 

 with, to form the external apertures. Further, we have seen 

 that the " body-cavity " of the earthworm is a true coelom, formed 

 in the embryo by the hollowing out of a number of paired 

 mesoblastic blocks or somites, and that it does not contain 

 blood, but a colourless, corpusculated fluid quite distinct from 

 the red corpusculated blood contained in a system of closed 

 blood-vessels. Lastly, we have seen that, although the earth- 

 worm has no obvious sense organs, no respiratory plates or 

 processes, no outgrowths or appendages of the body subserving 

 locomotion, that all these things are present in those marine 

 worms which are classed as Polychceta. We have now to 



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