ORGAN- SYSTEMS COMPARED 



metameric segmentation is observed, or even but partial traces of 

 it, can be discovered. Whilst it is certainly not necessary to 

 suppose that metameric segmentation is actually derived from an 

 arrested formation of strobilated buds which at one time were set 

 free, it is nevertheless tolerably certain that the fundamental 

 property of the organism is the same in both cases, bud-strobilation 

 and metameric segmentation, and that whilst (whether it takes 

 the form of antimerism or metamerism, or paramerism) we may 

 indicate the exhibition of this property by the name " merogenesis," 

 we can, with advantage, distinguish the clear and well-marked cases 

 of repetition of " meres " as " eumerogenesis " (e.g. Lumbricus and 

 Taenia, Agalma and Eudendrium), whilst the blurred and obstructed 

 cases, such as are furnished by the Vertebrates, the Chitons, the 

 Nemertines, and the imperfect antimeres of Holothurians are 

 spoken of as cases of " dysmerogenesis." The cases of eumero- 

 genesis are divisible into those resulting in separation, with or 

 without completion of parts, and those persisting as aggregations 

 with more or with less completeness and differentiation of the 

 " meres." 



The cases of dysmerogenesis are more difficult to analyse. 

 Their obscurity and incompleteness may be due to re-integration 

 following upon an earlier historical condition of eumerogenesis, 

 of which there is now no direct evidence (Chiton, Nautilus), or 

 they may be cases in which merogenesis sets in at an early stage 

 of individual growth and development, but has never in any 

 ancestral form persisted into adult life. In the last-named cases 

 merogenesis has never been more than a transient phenomenon 

 affecting the early stages of the individual, though it leaves 

 obscure and puzzling results of its existence which persist even 

 when full development is attained (? Vertebrates). 



7. CONCERNING THE CCELOM. 

 (a) Its historic definition. 



We designate by the name " ccelom " the cavity in Vertebrate 

 animals often called the pleuroperitoneal cavity, to which Haeckel 

 (see historical note below) originally applied the name, and for 

 which he invented it. We further, as a necessary result of mor- 

 phological theory, designate by the same name "ccelom" the 

 cavity or organ in other groups of animals which we consider to 

 be genetically identical with the primitive pleuroperitoneal cavity 

 of Vertebrates. " Ccelom " is not a term to be used for any and 

 every body-cavity other than the gut (as some eminent writers 

 seem to suppose), but definitely designates a morphological element 



