MOIv^OGRAPH OF THE EXISTING CRINOIDS. 93 



With the posterior arms of many of the comasttrids the case is quite diiferent. 

 P'rom the twenty-fifth brachial onward they taper very rapidly, and in Covianthus 

 parviciiTa, instead of reaching a length of 145 mm. as the anterior arms with some 

 150 brachials may do, they have only some 80 brachials, and rarely attain a greater 

 length than 60 mm. or 70 mm. At the same time their terminal pinnules are little, 

 if at all, longer than those of the middle portion of the arm, and the center of the 

 dorsal half of each of their segments is occupied hj a dark brown, egg-shaped body 

 of a peculiar celhdar nature, a so-called spherade, which Carpenter was inclined 

 to consider a sense organ (fig. 103-1, pi. 12). The pinnules of the last few segments 

 decrease very slowly in size, and the arm ends in an axillary which bears two 

 pinnules of the ordinarj' character, each provided with the brown ovoid bodies or 

 spherodes (part 1, fig. 47. p. 81). 



Between these two kinds of arms, the long anterior with a wide ambulacra! 

 groove and a well-developed respiratory apparatus (fig. 1038, pi. 12) and the short 

 posterior with a closed groove and no external respiratory apparatus (fig. 1037, 

 pi. 12), all possible forms of transition occur. 



In C omanthus parvicirra, as a general rule, more or fewer of the anterolateral 

 arms, C^ and E., are tentaculiferous, but they never reach such a great length as the 

 anterior arms, and their terminal pinnules are by no means so long and slender. 

 At the same time the posterolateral arms, C, and Ej, although generally non- 

 tentaculiferous, have, except in rare cases, a more or less open groove for the greater 

 part of their length, which, while greater than that of the posterior arms of the 

 radius D, is less than that of the anterolateral arms Cj and E^, and their pin- 

 nules increase slightly in length from the middle until near the end of the arm. 



Carpenter thus classifies the arms of C omanthus parvicirra: 



Anterior^ on radii A and B; 120-150 brachials; pinnules increasing in length 

 to the terminal, which are very long and slender ; tentaculiferous. 



Anterolateral, on C^ and E„; 100-120 bracliials; terminal pinnules long and 

 slender ; tentaculiferous. 



Posterolateral, on C„ and Ej ; 80-100 brachials ; terminal pinnules stout and 

 rather longer than the middle pinnules ; pinnules usually with spherodes and nar- 

 row ambulacral grooves, but nontentaculiferous. 



Posterior, on radius D; 60-80 brachials; terminal pinnules stout, but shorter 

 than the middle pinnules; spherodes are present; usually no ambulacral grooves; 

 nontentaculiferous. 



Carpenter noted that another difference between the anterior and the pos- 

 terior arms is that the gonads on the latter are far more developed than those on 

 the former; not only is their number greater, although the total number of pinnules 

 on a posterior arm may not be much more than half that on an anterior arm, but 

 they attain a very much greater size, the proximal and middle pinmdes of an 

 anterior arm being very much less swollen than the corresponding pinnules of a 

 posterior arm. 



In larger species of Comasteridte Carpenter noticed that sometimes all the arms 

 might be grooved and bear tentacles, yet the posterior might be considerably 

 shorter than the others. In one species he noticed that, though all the arms are 



