HISTORY OF THE TERM CCELOM 15 



establish a theory of the uniform origin of .the renal organs in 

 all Ccelomoecela from nephridia, is that the nephridia, though 

 primarily superficial and ectodermic, do acquire an internal open- 

 ing into the coelom in the Chaetopoda. The funnel-like internal 

 mouth (nephridiostome) which they often but not always develop 

 under these circumstances is part of the same chain of cells which 

 form the nephridial tube. Moreover, Goodrich has shown that the 

 nephridia which thus penetrate to the coelom in Chaetopoda, 

 may acquire most intimate relations to the coelomoducts and their 

 caelomostomes. In the marine forms (Polychaeta) this associa- 

 tion leads to the formation of complex organs consisting partly 

 of coelomoduct with ccelomostome and partly of nephridium. 

 These remarkable facts have only recently come to light, and 

 readily explain the confusion which has hitherto prevailed between 

 the ectodermal nephridia and the ccelomic coelomoducts. 



8. THE HISTORY OF THE TERM CCELOM AND THE THEORIES 

 CONNECTED WITH IT. 



(a) From Haeckel, 1872, to the Hertwigs, 1881. 



The word " ccelom " was introduced into morphological science 

 by Haeckel in 1872. In the first volume of his " Kalkschwiimme," 

 p. 468, Haeckel writes as follows : "Die wahre Leibeshohle welche 

 bei Vertebraten gewohnlich Pleuroperitonealhohle genannt wird, 

 und fur welche wir statt dieses neunsylbigen Wortes die 

 bequemere zweisylbige Bezeichnung Coelom (TO KoiAw/ia, die 

 Hohlung) vorschlagen, findet sich nur bei den hoheren Thierstam- 

 men bei den Wurmern, Mollusken, Echinodermen, Arthropoden 

 und Vertebraten." 



According to the theoretical conception which was justified by 

 the imperfect knowledge of embryological facts of that time, 

 Haeckel regarded the coelom as a space formed by a " split " in 

 the blastoderm dividing the middle cell-layer into two secondary 

 layers. According to this view the outer of these, the dermal 

 fibrous layer (Hautfaserblatt), adheres to the ectoderm to form 

 the fibrous and muscular layer of the body-wall ; the inner, the 

 intestinal fibrous layer (Darmfaserblatt), adheres to the endodermal 

 lining of the gut to form the fibrous and muscular part of the 

 gut-wall. It was natural and justifiable to provisionally identify 

 with the Vertebrate split-space thus formed and distinguished as 

 " the coelom " the chief cavity lying between gut-wall and body- 

 wall in Mollusca and Arthropoda, as well as the similarly situated 

 cavities of Chaetopoda and Echinoderma. The hypothesis as to 

 the origin of the coelom was that it was formed by the accumula- 

 tion of nutrient fluids which passed through the wall of the 

 alimentary canal. Thus Haeckel erroneously identified the dis- 



