314 COELOMATA. 



the facts before the reader, our treatment of it must be more or less 

 dogmatic. 



The coelom presents the fullest and, what we may call most 

 typical development in the dibranchiate Cephalopoda. Here it is 

 divided into three parts, all of which communicate. These parts are, 

 (1) the genital sac or gonadial coelom, from the epithelial walls of 

 which the reproductive cells arise ; (2) the viscero-pericardial sac or 

 perivisceral coelom, which has the functions of a typical body-cavity 

 for the heart and certain other viscera* (3) the renal sacs or nephridial 

 coelom, the epithelial walls of which' secrete the nitrogenous waste, 

 thus performing the functions of kidneys. 



As stated above all these sections of the coelom are in communi- 

 cation, the gonadial with the viscero-pericardial, and the viscero- 

 pericardial with the nephridial. Moreover, the gonadial and nephridial 

 open directly to the exterior, the former through the generative ducts, 

 and the latter through the nephridiopores. From this it would 

 appear that the coelom, in addition to its mechanical relations, has 

 two most important functions : the one of these is to bud out the 

 reproductive cells, and the other to secrete the nitrogenous waste. 

 These functions are always discharged by the coelom in the Coelomata. 

 In certain forms, e.g., Peripatus, elasmobranch fishes, Cephalopoda, 

 some Gastropoda and the Annelida, this is perfectly obvious ; but in 

 most Arthropods, and in some Vertebrates and Molluscs it is a 

 matter of inference, and might possibly be disputed by those zoologists 

 who have not followed recent morphological research. We, however, 

 have no hesitation in laying it down as a general law that in all 

 Coelomata the functions of producing the reproductive cells and 

 secreting the nitrogenous waste are discharged by this organ which 

 we now call Coelom. 



With regard to the origin of the coelom and its relation to other 

 organs, we have here only the following statements to make. 



The coelom is derived from the enteron ; it is a part of the enteric 

 cavity, which has, in all Coelomata, lost its connection with that 

 portion which constitutes the alimentary canal in the adult, though 

 it has retained its connection with the enteron, thus showing its 

 enteric origin, in the young stages of certain groups, e.g., the 

 Echinodermata, Chaetognatha, Brachiopoda, Enteropneusta, and in 

 Amphioxus. In the one division of the Coelenterata, viz., the 

 Actinozoa, there is an incipient coelom in the enteric pouches, on 

 the walls of which the reproductive cells are formed. The Coelen- 

 terata, however, are not Coelomata; they have no organ distinct from 



