72 SMITHSONIAN INSTITUTION 



From the foregoing observations we see that the position and movements 

 of the anal plate are not governed by its connection with other plates of the 

 aboral side, but that they depend upon the shifting and development of the gut, 

 to which it is at an early stage attached. And we can readily trace in the 

 movements of the plate thus indicated striking analogies to some of the anal 

 conditions observed among the paleozoic genera. 



Now we have in two great living types of the comatulids just such differ- 

 ences in the position of the anus — though in less degree — as we have supposed 

 to occur among the ancient forms; viz., that of Antedon. being excentric, while 

 that of Comactinia, Comanthus (Actinometra) , and their allies, is substan- 

 tially central. The excentricity of Antedon, while producing an anal plate in 

 the larval skeleton, is not sufficient, or sufficiently persistent, to affect the form 

 of the calyx in the adult, which in both types has perfect pentamerous 

 symmetry. 



We may also be warranted in supposing, from the observed course of 

 development of the anal structures in the larva of the comatulids as above 

 described, that the Taxocrinus plan represents the earliest and most primitive 

 form, and that the modifications of this in paleontological time were those 

 which tended toward the disappearance of the row of anal plates, with its 

 border of perisome, and the substitution for a time of regular calyx plates 

 suturally connected with the adjacent rays as in the Camerata. Hence the 

 final outcome of the struggle from Paleozoic to Recent time was the survival 

 of the original plan, and the suppression of the modified one. 



3. The third modification of the anal structures is that in which the anal 

 plate has altogether disappeared, and it is merely a further extension of one of 

 the preceding. This was accomplished in the Carboniferous, where in the 

 genus Metichthyocrinus we have a crinoid with perfect pentamerous sym- 

 metry, so far as the external skeleton shows. A genus in the Devonian, 

 Synaptocrinus, reaches this stage, and two others in the Carboniferous — 

 W achsmnthicrinus in which traces of bilateral symmetry can be seen in the 

 slightly larger size of the posterior basal, and Nipterocrinus in which the pen- 

 tamerous symmetry seems to be perfect. This modification apparently did not 

 greatly influence the history of the group. 



c. The first modification in the brachial system — i. e., in the number of 

 primibrachs — has already been described and discussed as to its details. There 

 can be little doubt that the primitive form in this respect was that with two 

 primibrachs. It is that of the comatulid larva, and it prevailed almost ex- 

 clusively in the Silurian. The few cases in which there is but one primibrach 

 may be explained by the syzygial union of two of the primitive brachials, just 

 as happens in some species of the living Antedon, without changing the funda- 

 mental plan. The addition of another primary brachial simultaneously in all 



