METAMEEIC SEGMENTATIOK 61 



various distinctions. The appearance of a liead is at the same time 

 a result which affects the whole body^ for the body can now be 

 divided into two portions at least^ which differ in character. 



Metamerism of the Body. 



§ 51. 



The planning" out of the individual organism as a single struc- 

 tural entity is only characteristic of lower conditions of development, 

 whether permanent, as in nearly all Coelenterata and in the lower 

 classes of Worms, or transitory, as in the higher divisions of the 

 Animal Kingdom. Simultaneously with the growth of the body to a 

 considerable length, we observe the beginning of the division of 

 the organism into separate segments, following one on another, 

 noticeable externally through the occurrence of separating constric- 

 tions, or through the regular distribution of appendicular structures 

 or processes of the body, internally represented by the arrangement 

 of the organs in the distinct successive compartments of the body. 

 We term this segmentation of the body Metamerism; the separate 

 segments are metameres. The metamerism which thus breaks 

 up the body is only a further example of differentiation. From the 

 primitive homogeneous indifferent body a heterogeneous, diversified 

 body is developed, and the separate metameres differ from one 

 another ; not only are they something new in comparison with the 

 earlier condition, but they are also — notwithstanding their resem- 

 blance one to another — different from one another, owing to the 

 position which each occupies. 



Metamerism is not in all cases, where it is perceptible, exhibited 

 with equal clearness. Sometimes it is apparent in this or that 

 organ, or system of organs, more than in another, and, again, in 

 other organs may be altogether wanting'. It is easy to recognise 

 very various conditions of the commencement and of the incomplete 

 carrying out of the process. Where we find it in fullest develop- 

 ment it dominates the whole organism, and is exhibited in all organs ; 

 so that each metamere possesses its individual system of organs, 

 and particular systems of organs common to all metameres present 

 a special differentiation of their structure in each metamere (ventral 

 ganglion chain). In this manner the organism becomes built up of 

 many component parts. And hereupon we have to take note of 

 conditions in which independent importance is acquii-ed by the 

 metameres. Little by Httle a metamere, in virtue of the elaboration 

 of its own set of organs, ceases to be dependent on the total 

 organism, emancipates itself from the commonwealth, and gains the 

 capability of leading a fi^ee existence. To this many phtenomena are 

 traceable, which are usually called gemmation (Worms). 



