MUSEUM OF COMPARATIVE ZOOLOGY. 29 



that it antedates all the plates of the actinal hemisome. Agassiz sup- 

 posed it to be formed after the other ten abactinal plates, and Ludwig * 

 does not say that it is or is not formed at that time. 



The time of formation and mode of growth of the terminals, at the tips 

 of the arms, seems to be the same as already described by A. Agassiz 

 and Ludwig. Agassiz found them to form before the genitals, which 

 is true also in my larvse. I cannot verify the statement that the ter- 

 minals ever fuse with each other, as described by Agassiz. The 

 arrangement of spines on the terminals differs somewhat from those of 

 Asterina. 



According to Ludwig (p. 50, op. cit.) one of the interradials (geni- 

 tals) precedes in time of formation and size the other genitals and 

 the terminals. This is the genital which later forms or fuses with 

 the madreporic plate. This predominance of the genital contiguous 

 to the madreporic opening was not noticed in Asterias, although the 

 relative distance and general situation of this plate as compared with 

 the madreporic opening are about the same as Ludwig describes for 

 Asterina. 



My observations on the growth of the plates of the abactinal region 

 of the arms resemble those recorded by A. Agassiz, Loven, Ludwig, and 

 Viguier. The calcifications of the body in the abactinal hemisome also 

 resemble, with some exceptions, those already described. Accordiug to 

 the first author (p. 37), in an early condition after the eleven abactinal 

 plates were formed " the whole of the abactinal surface has become 

 coated with a very fine granular deposit of limestone." This formation 

 was not seen in the specimens of Asterias which were studied. 



The observations on the mode of formation of the oral ambulacrals 



* His youngest stages show eleven plates, and in the text he speaks of them 

 as if the " Centrale," dorsocentral, was synchronous in formation. (See Entwick- 

 lungsgeschichte der Asterina gibbosa, Forbes, Zeit. f. Wiss. Zool., Vol. XXXVII.) 



The homology of the calcifications of the pluteus of Ophiurans and Echinoids 

 witli calcifications in the stem of the Crinoids would seem far-fetched. The spines 

 of the pluteus are secondary developed structures, and it is believed by some that 

 they have no phylogenetic significance. The fact that they are wanting in the 

 brachiolaria of Asterias would look that way, but in Amphiura they are repre- 

 sented before the larva leaves the mother. It is possible in this instance to be- 

 ,lieve either that Amphiura is descended from a genus which had a pluteus with 

 spines, and in its abbreviated metamorphosis the rudiment of the spines only re- 

 mains, or that the plutean spines show relations with other groups outside the 

 Ophiurans. The latter conclusion does not appear absurd, and it may be possible 

 later to show that there is an homology between the stem of a Crinoid and the 

 plutean spines of an Ophiothrix. 



