450 ANNELIDA. 



excepted) becomes more or less broken up into a system of com- 

 municating spaces. 



The renal organs, called nephridia, are portions of the coelom, 

 though this is not so obviously the case as in certain other groups 

 (e.g., Vertebrata, Peripatus, Molluscd). Still they almost always 

 retain a communication with the rest of the coelom in the ne- 

 phrostome or ciliated coelomic funnel (Chaetor/aster and one or 

 two other forms excepted). They almost always have a tubular 

 form, and open externally on the ventro -lateral surface of the 

 body. As might be surmised from the condition of the coelom 

 in the embryo, they share in ~the segmentation of the body ; and, 

 moreover, with regard to them it should be noted that they very 

 rarely (Ecliiuroidea, Sternaspis, Ohlorhaemidae) lose their relation to 

 the segmentation of the body. As a general rule there is a single 

 pair of nephridia in each segment, but this rule is sometimes departed 

 from in the Chaetopoda, where there may be more than one pair in a 

 segment (Perieliaetidae, Acanthodrilidae, Capitellidae.} 



In a few cases some of the anterior and posterior nephridia 

 shift their external openings from the outer surface of the body 

 into the anterior and posterior end of the alimentary canal. This 

 is seen in some Oliyochaeta (so-called pepto-nephridia), and in the 

 case of one posterior pair in the Echiuroidea (anal vesicles). 



The generative organs are always products of the coelomic walls, 

 being thickened patches of the coelomic epithelium. Occasionally, 

 in the male more often than in the female, the generative section 

 of the coelom is cut oft* from the rest by special membranes. This 

 is seen in the sperm-reservoirs of some Olir/ochaeta. 



The generative ducts open into the coelom, and have, of course, 

 a tubular character with ciliated internal openings. Sometimes 

 they are distinct from the nephridia (Oh'gochaeta), but, as a rule, 

 some or all of the nephridia are used for the exit of the generative 

 products, thus combining a renal and generative function. This 

 association of the generative and excretory functions in the carrying 

 apparatus is, of course, to be connected Avith the fact that these two 

 functions are discharged by the same organ, viz., the coelom ; it is 

 a feature characteristic on the whole of all coelomate animals (cf. 

 Vertebrata). 



The Annelida are for the most part aquatic animals, but terrestrial 

 forms are known (earth worms). 



Their eggs are sometimes laid, several together, in a cocoon 

 (Hirudinea and Oligochaeta), but, as a general rule they are 



