METAZOA 123 



triploblastic animals, so that these organs are unaffected by the move- 

 ments of the body wall and are able freely to perform movements of 

 their own, may be either coelomic or haemocoelic, but is usually 

 coelomic. In the Arthropoda, where the perivisceral function of the 

 coelom is entirely usurped by the haemocoele, the former space is 

 reduced to small cavities in the gonads and excretory organs. 



In animals which possess a coelom, the gonads are derived from 

 its walls, and either the germ cells are shed into a coelomic peri- 

 visceral cavity or the gonad itself contains a cavity which is a separated 

 portion of the coelom. 



The coelom communicates with the exterior. The communication 

 is usually made through organs belonging to one or other of the types 

 known as "nephridia" and "coelomoducts", though it occasionally 

 takes place through openings of other kinds, such as the dorsal pores 

 of the earthworm and the abdominal pores of fishes. 



Nephridia and coelomoducts are organs which meet the need for 

 the passage to the exterior of products of organs derived from or 

 imbedded in the mesoderm. Their characteristic features are as 

 follows : 



(a) The nephridia! system is primarily an organ which serves the 

 mesenchyme, though it may come to lie in the coelom, and in certain 

 annelids communicates with that space. It consists of intracellular 

 tubes of ectodermal origin, usually branched and bearing at the end 

 of each branch a solenocyte or flame cell {see p. 177). It may be con- 

 tinuous or divided into segmental units, the nephridia. Water 

 containing excreta is shed by the protoplasm of the tubes, and passes 

 out in the current set up by the action of the flame cells or by cilia. 



(b) Coelomoducts are mesodermal passages which open at one end 

 to the exterior and at the other usually into the coelom, though the 

 coelomic opening may lead only into a minute vesicle of the coelom, 

 or even be lost altogether. They may (i) be solely excretory, the 

 excreta being shed into them by gland cells in their walls, or borne 

 into them by a current of fluid from the coelom through the 

 coelomic opening of the organ, or derived from both these sources^; 

 (2) combine excretion with the function of conducting the germ 

 cells to the exterior; (3) be simply gonoducts, which was perhaps their 

 original function. 



Many annelida possess compound excretory organs formed by the 

 union in various ways of nephridia with coelomoducts or other 

 mesodermal elements (see p. 243). In such cases the nephridia 

 acquire a communication with the coelom, and excreta or germ cells 

 may pass from it through them. In other groups, as in some 



^ Watery excreta are sometimes concentrated by absorption during their 

 transit through the passages of the organs of excretion. 



