212 PROCEEDINGS OF THE ACADEMY OF [-M^^il, 



cui^ily tlelimitcd, since they are but local specializations of a whole. 

 And tlic relative kinds of parts intergrade, so that it is purely- 

 arbitrary to distinguish, e. f/., between organ and organ system, 

 between raetaniere and region, etc. Though it is necessary to give 

 distinctiue names to particular parts, for purposes of description, 

 it must be kept in mind that the parts or ' ' organs ' ' in being not 

 sharply delimitable are purely arbitrary distinctions. Thus, an 

 ' ' organ ' ' is more of a concept than an organism, since it is not so 

 perceptibly delimitable; though less of a concept than a species, 

 since the connection of its components is perceptible. 



In one organization we find the general body plan, the major 

 parts which compose this, and the minor parts Avhich compose the 

 latter. In a radiate body plan, as shown by a star-fish, the body is 

 composed on the principle of converging rays, the autimeres; each 

 of the latter is bilaterally symmetrical, with a distinction between 

 dorsal and ventral, and contains a series of ' ' organs ' ' ; and each 

 of these organs has its minor parts. In a trematode there is a 

 bilaterally symmetrical body plan, without antimeres; in an 

 annelid, a bilaterally symmetrical united with a metameric, the 

 larger parts being here the segments or metameres. Other body 

 plans are the monaxonic, where there is more or less strictly radial 

 grouping of the parts along one long axis ; and the homaxouic, very 

 rarely found, where all the axes passing through the central point 

 are of equal length. But still other distinctions of body plans are 

 possible. Thus : that in which the two ends of the body are dissimi- 

 larly developed, as head and tail, which might be known as the 

 antipolar; where the body axis is straight and where it is bent, 

 (Homaxonic and Heteraxonic, Hatschek) ; and various others which 

 may be arbitrarily distinguished. Therefore there are many kinds 

 of general body plans." Of the major components of these body 

 plans we may distinguish antimere and metamere, anterior and pos- 

 terior, right and left, dorsal and ventral, etc. The major compo- 

 nents of the latter are what are generally called ' ' organs, ' ' such as 



"Though we use here the term body plan, we cannot any longer 

 restrict its use to that of Cuvier, von Baer, and L. Agassiz ; the four 

 fundamental body plans distinguished by these naturalists represented 

 to them four separate ^ais of creation, which were unbridgeable. In the 

 li"-ht of modern Phylogenj' we know that animals were not created separ- 

 ately in four separate moults, and know that gradations are found between 

 the ditierent body " plans " or " types." 



