1902.] NATURAL SCIENCES OF PHILADELPHIA. 219 



relationships. The body plan is easily influenced by the mode of 

 life, as demonstrated by Lang and others ; Lang has shown that 

 radiality and sessility are often associated, a creeping motion 

 and bilateral symmetry. Perhaps such a relation would be better 

 expressed in the statement : the position of the mouth opening is 

 determined by the mode of life, and the relative arrangements of 

 the other parts become modified by change in its jDOsition. And 

 there is another relation, also referable to the mode of life, namely, 

 modifications of the arrangements of parts caused by the develop- 

 ment of firm skeletal structures, as the shell in the Mollusca. 



In general, then, we may conclude that the modes of arrangement 

 of the parts of the organization, which are known as radial and 

 bilateral symmetry, the relation of dorsum to venter, etc., are 

 characters of small degree of conservatism. But it is difficult to 

 decide on the morphological value of metameric arrangement. For 

 metamerism cannot be explained, as other modes of arrangement 

 may be more or less satisfactorily, by the mode of life, but would 

 rather appear to be referable to intrinsic growth methods ; and it 

 also does not become readily changed by variation in mode of life. 

 Metamerism is also in no way a function of comj^arative size or 

 length of the body, but would seem to be the consequence of 

 primarily the mode of formation of the mesoderm (E. Meyer). 

 Metamerism is more conservative than other body plans, and so 

 must be given a greater value in classification. Thus the group of 

 the Articulata is nuich better founded than the earlier postulated 

 group of the Kadiala, for it is based upon a more conservative 

 mode of arrangement of the parts. And as generally held, meta- 

 merism of the inner parts is of much more morphological value 

 than metamerism of the tegumentary structures, on account of the 

 greater conservatism of the former. 



The parts of the organization which furnish the best basis for 

 classification are the organs, as this term is generally used. These 

 parts may not reflect so fully the organization as a whole as do 

 larger parts which they compose, such as antimere, metamere, 

 right aod left sides, head and trunk, etc. ; but comparative 

 anatomy has shown that they are, on the whole, more conservative, 

 and on this account they are to be preferred. Their study, unlike 

 study of cell or tissue components, gives a synthetic view of the 

 whole orfranization. 



