218 rROCEEDlXGS OF THE ACADEMY OF [April, 



Echiuotlermata there is a general radiate plan of the larger parts 

 (antinieres), but each antimere may be bilaterally symmetrical, 

 and the intestine, unlike the other organs, is not rayed. In the 

 Anthozoa there is at once bilateral symmetry and radiality. In the 

 Turhellaria are forms, which, in respect to the intestinal branches, 

 are more or less clearly radial, and at the same time, with regard 

 to the excretory and genital systems, more or less bilaterally sym- 

 metrical ; and the Trematoda have a radial arrangement of certain 

 organs, an unpaired (monaxonic) and bilaterally symmetrical 

 arrangement of others. The metamerism of the Articulata does 

 not extend to the intestine; and in the Gasteropoda a true bilateral 

 symmetry may be found in the parts enclosed witliin the foot, while 

 such symmetry is to great extent obliterated in the spirally wound 

 superior portion of the body. There is, in fact, no animal known 

 in which all the parts are perfectly bilaterally symmetrical, or all 

 perfectly metameric, and only in certain Cnidaria does there seem 

 to be perfect radial symmetry.^' The facts being so, Ave are not 

 justified in considering as of much value the plan of arrangement 

 of the parts, or what is known as the " body plan." And an- 

 other well-known illustration bears this out. Throughout the group 

 of the Platodes (comprising the Turbellaria, Cestoda and Trema- 

 toda) there is a uniform type of excretory organs (protonephridia); 

 but the external opening of these organs may be very differently 

 situated in different organisms, and in closely related forms (of the 

 Trematoda) these organs may be paired or unpaired. 



There is a further series of facts to show that the relative posi- 

 tions of parts may be very different in closely related organisms ; 

 thus, the mouth opening and the genital apertures in the Turbel- 

 laria; the mouth opening sometimes on the right, sometimes on 

 the left side in the Gastei'opoda (even in different individuals of 

 the same species) ; the position of the abdominal nerve ganglia in 

 the Diptera, etc. 



Such facts, which any comparative anatomist can mid ti ply, show 

 that the general plan of arrangement of the parts is usually far 

 from conservative, and hence, judged by the criterion of degree of 

 conservatism, is of little value for determining broader phyletic 



" Many of the Protozoa which were first regarded as of a perfect radial 

 type, have more recently been shown to be asymmetrical in some 

 respects. 



