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THE AMEBIC AX XA TUB A LI ST [Vol. XLIII 



transverse ridge into the dorso- ventral axis; hence, the 

 transverse ridges of the two primitive straight muscular 

 articulations coincide, and remain unchanged in the re- 

 sultant synarthry; a rotation of two succeeding oblique 

 muscular articulations 90° will, as their transverse ridges 

 are already at right angles to each other, keep them in 

 the same relative position ; projecting one of them upon 

 the other, the two transverse ridges form a right-angled 

 cross ; since the muscles and interarticular ligaments, be- 

 ing recessive when compared with the dominant dorsal lig- 

 ament, disappear, we get an articulation consisting of a 

 mass of dorsal ligament fiber crossed by radiating ridges. 

 This crossing of the two fulcra upon a single joint face 

 effectually prevents any movement at the articulation, 

 and thus we get the primitive syzygy. The multiplica- 

 tion of the radiating ridges is without doubt a secondary 

 development, though possibly four of them represent the 

 distal edges of the interarticular ligament fossse. The 

 interpretation of a syzygy as a combination of the liga- 

 ments of two oblique muscular articulations explains the 

 uniformly single condition of the syzygy, the absence of 

 pinnules, and the neutrality of the syzygy in regard to 

 pinnulation, this being brought about in the same way 

 as in the synarthry, the resultant of two straight mus- 

 cular articulations. 



The interpretation of synarthries and syzygies just 

 proposed involves a doubling up and merging together 

 of the elements of two muscular articulations. This, it 

 might well be argued, would be an occurrence improbable 

 in the extreme in a linear series of joints and articula- 

 tions. The ambulacral system of echinoderms, however, 

 is composed primarily of a double series of joints, plates 

 or whatever the elements may be, the first element alone 

 being single. Thus in the urchins the oculars stand at 

 the head of the double row of plates composing the 

 ambulacra ; and in the crinoids the radials stand at the 

 end of an ambulacral system composed, in many genera, 

 of two rows of plates, side by side, in biserial arrange- 



