CCELOM AND EXCRETORY ORGANS 13 



{d) Tlie Coelom and Excretory Organs. 



The physiological significance of an increase of size of the 

 original ca?lomic sacs is not difficult to suggest. Whether in the 

 presence or absence of a haemal system the accumulation of a 

 quantity of organised liquid in cavities (or in a single cavity 

 formed by the fusion of two or more original crelomic sacs) must 

 have considerable physiological significance. The coelomic fluid 

 and the coelomic epithelium, as well as the floating corpuscles 

 derived from that epithelium, acquire special properties and im- 

 portance over and beyond the original functions subservient to 

 the maturation of the gonadial cells. The mechanical significance 

 of this liquid-holding chamber and its erectile function, similar to 

 the erectile function of the archenteric cavities in such Anthozoa 

 as the Pennatulids, are noteworthy ; but the most important 

 develoj^ments of the co?lom are in connection with the establish- 

 ment of an exit for the generative products through the body-wall 

 to the outer world, and further in the specialisation of parts of 

 its lining epithelium for renal excretory functions. 



In the Enteroccela the generative products either escape by 

 rupture of the body-wall outwardly, or are liberated into the 

 archenteron, and so escape by the mouth. Even in the Entero- 

 ccela pores exist in many forms which permanently place the 

 peripheral parts of the archenteron in direct communication with 

 the exterior ; but these jDores do not serve as passages for the 

 generative products (aboral pore of Peachia, tentacle pores of 

 Actinians, and polar pores of Ctenophora). Though in some 

 cases the generative products of the Coelomocoela escape from the 

 coelom by rupture of the body-wall, yet the existence of paired 

 apertures right and left, serving for the exit of the genital pro- 

 ducts from the ca'lomic sacs, must be regarded as a very early 

 feature in the history of the Coelomocoela. These apertures are 

 not formed by an invagination of the ectoderm, but by an out- 

 ward, often tube-like growth of the coelom itself. They become 

 specialised in many groups in the form of more or less coiled 

 canals, and require to be recognised by a distinct name. I propose 

 to call them coelomoducts.^ Frequently they are furnished with 

 trumpet-shaped or funnel-like internal mouths. Such funnels are 

 termed ca?lomostomes. They exist where the coelom is large and 

 spacious, and the gonad (ovary or spermary) is not specially en- 

 closed in a duct-forming sheath, shutting it off" from the rest of 

 the coelom (a shutting-off" which does take place in the Leeches 

 and Eudrilid Earthworms, and also in Echinoderms and many 



' There is no convenient Greek eciuivalent for " duct," and I hold that we are 

 therefore justified in coining such hybrid words as "coelomoduct," "gonoduct," and 

 " uroduct." 



