MONOGRAPH OF THE EXISTING CRINOIDS. 



95 



In Comatula pectinata all the arms may be grooved, or from one to six may be 

 ungrooved; and the same is true in C. micraster, though in this little species the 

 average frequency of ungrooved arms is rather higher than in C. pectinata. In 

 both these species the difference in length between the grooved and the ungrooved 

 arms is often very great, the former being sometimes as much as three times as long 

 as the latter (fig. 163, p. 86). 



In the multibrachiate species the character of the division series giving rise to 

 these ungrooved arms is commonly different from that of the division series sup- 

 porting the ordinary grooved arms, this difference Ijeing in the direction of greater 

 simplicity of structure evidenced by the general substitution of division series of 

 two elements for those ordinarily of four. Sometimes in those species in which, as 

 in Comanthina schUgelii and Comaster belli, the IIIBr series are normally 4(3+4) 

 internally and 2 externally, there is a reversion of this order on the posterior arms, 

 the arrangement here being then 2 internally and 4(3+4) externally, an arrange- 

 ment duplicated in Ilimerovietra and in the very many armed species of Zygometra. 

 The radials from which these modified arms arise are sometimes dwarfed, 

 though usually they appear to be no different from the others. The length and 

 thickness of the division series is, however, often considerably less (fig. 1017, 

 pi. 7). This diminution in size is not gradual, but very sudden, so that the 

 ossicle following the radial appears disproportionately small, as if the postradial 

 series had been broken off at the radial and subsequently regenerated. 



Ludwig noticed that the terminal segments of the arms and pinnules of 

 Heliometra glacialis are devoid of ambulacral grooves, and P. H. Carpenter 

 found that this obliteration of the ambulacral grooves is brought about by the 

 approximation and fusion of the elevated folds of perisome at their sides, corre- 

 lated with a loss of tentacles and associated structures, just as in the Comasteridse, 

 the only difference being that it takes place much farther from the disk. 



Within the same or closely related species the proportionate length of the 

 brachials and arms is to a large degree correlated with the proportionate length 

 of the cirrals and of the cirri. This is well illustrated by such species as occur 

 in localities where the temperature and salinity are above or below that ordi- 

 narily or typically normal for the genus or family. Thus in the warm Adriatic 

 and Mediterranean Seas we find Antedon adriafica (part 1, fig. 106, p. 171) and 

 A. mediterranea (part 1, fig. 105, p. 169), both of which, especially the former, 

 are remarkable for the length and slenderness of the arms, as well as for the 

 length and slenderness of the cirri and cirrals, characters by which they are 

 sharply separated from A. petasus (part 1, fig. 103, p. 165) and A. hifida (part 1, 

 fig. 104, p. 167) of the colder waters of the east Atlantic, which, especially the 

 former, possess relatively short and stout arms, and short, stout, strongly curved 

 and short-seomented cirri. On the muddy bottoms in the shallow, warm, and 

 fresh Java Sea many of the wide-ranging East Indian types, such as Cap'dlaster 

 sentosa, C. muUiradinta, Comatula purpurea, Comaster fruticosus, Heterometra 

 producta, Oxyinetra fi.nschii, and Di^hrometra fageJlata, are represented by par- 

 allel types, in which the cirri, and usually also, though commonly to a somewhat 

 lesser degree, the arms, are exceedingly elongated and very slender. On the 



