GENERAL LAWS OF ANIMAL STRUCTURE. 2 7Q 



radial arrangement of the nervous system is shown in 

 Fig 67, page 92. The other systems will be illustrated 

 hereafter. 



Comparison of this with Other Types.— Com- 

 paring this with other types, in both vertebrates and 

 articulates we have segments repeated in a linear series, 

 and therefore an anterior and posterior extremity; but in 

 this we have segments repeated in a circular series, and 

 therefore no beginning and no end, no anterior and no 

 posterior extremity. Again, in all other types, includ- 

 ing mollusca, we have all the organs, especially those of 

 animal life, repeated on the two sides of a median plane 

 — i. e., bilateral symmetry ; in this one we have repetition 

 of organs about a central column — -i. e., radial symme- 

 try. In the highest radiates alone are found the distinct 

 beginnings of a bilateral symmetry. 



In this also, as in the other two segmented types, the 

 completeness of the segmentation, the number of re- 

 peated parts, and their similarity are greatest in the 

 lower part of the scale ; and as we rise the segments 

 become more and more dissimilar by modification for 

 various functions. But the obscuration by modification 

 is less conspicuous in this type, because, taken as a whole, 

 the animals of this are less highly organized. 



4. PROTOZOA. 



Among these, of course, there is as yet no distinct 

 plan of structure (for they consist of a single cell), and 

 structure such as we have been speaking of is produced 

 by differentiation of the elements of a cell aggregate. 

 These lowest animals may be regarded as the living 

 stuff out of which the different types were constructed — 

 the trunk from which diverged the four great branches 

 (Fig. 158, page 248). There is no room for homology 

 here. 



