134 THE INVERTEBRATA 



which is derived, as we have seen, from the endoderm. This cavity 

 has various forms, but is rarely tubular and never possesses a heart. 

 Usually it constitutes one or more large perivisceral spaces around the 

 heart, alimentary canal, and other organs. It will be noted that the 

 perivisceral cavity which surrounds the internal organs of most triplo- 

 blastic animals, so that these organs are unaffected by the movements 

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

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

 (Fig. 107 a-c). In the Arthropoda, where the perivisceral function 

 of the coelom is entirely usurped by the haemocoele {d~g), 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 perivisceral 

 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 nephridial 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 is for the most part intra- 

 cellular, and consists of tubes, often, at least, of ectodermal origin, 

 usually branched and bearing at the end of each branch a solenocyte 

 or flame cell (see p. 202). It may be continuous or divided into seg- 

 mental units, the nephridia. Water, probably 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 (see p. 141) ; 

 (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 



