MONOGRAPH OF THE EXISTING CBINOIDS. 117 



If there are only 10 arms, the possession of exceedingly short discoidal brachials 

 denotes the. family Himerornetridse (the genus Amphimetra) (see fig. 86, p. 141) ; tin- 

 presence of paired or tripled dorsal spines or of a broad transverse ridge on the 

 outer cirrus segments denotes the family Colobometrida- (see figs. 346-348, p. 289, 

 and 349-352, p. 291); while if none of these features are shown the specimen belongs 

 to the Antedonidse. 



This method of procedure for the determination of the various comatulid 

 groups is the most certain, though it is very unnatural in that it separates widely 

 genera belonging to the same family, and is based more or less upon characters 

 which, though very obvious and perfectly reliable, are systematically and morpho- 

 logically of but slight importance. A single family of comatulids may contain 

 species with from 5 to over 100 arms and therefore of radically different appearance, 

 though practically identical in fundamental structure, and it therefore becomes 

 necessary to handle the comatulid species in a somewhat arbitrary way unless we 

 wish to have recourse in each case to elaborate dissection in the determination of 

 the species. 



The young of the comatulids are as yet very imperfectly known, and the 

 identification of specimens of multibrachiate species in the 10-armed stage is 

 involved in no little difficulty, especially where there is but little specific differentia- 

 tion in the oral pinnules as in the species of Comasteridse. But in the echinodcrms 

 the adult skeletal characters are as a rule assumed at an extraordinarily early 

 age, and the crinoids form no exception to this generalization. In the 10-armed 

 species the young usually resemble the adults sufficiently so that a close com- 

 parison, assisted by a judicious use of circumstantial evidence, is as a rule enough 

 to make the identification reasonably certain. In the young all the ossicles are 

 much elongated, the lower pinnules may be more or less deficient, the radials are 

 thin and broad, the basals may form a closed ring about the calyx as in the adult 

 Atelecrinus, while the cirri, arms, and pinnules have fewer segments, and those 

 more generalized and usually more elongated than those of the adults. The plating 

 of the disk may be highly developed at a very early age, as in the species of Calo- 

 metridse, in Comactinia and in Catoptometra; or in certain species in which it is well 

 developed in the adults it may be quite lacking in the young, as in some of the 

 Tkalassometridse. Side and covering plates, or the latter alone, are usually evident 

 at a very early age. 



All young comatulids have the division series uniformly narrow and well 

 separated, no matter how broad they may become later in life, while the carination 

 of the brachials and the prismatic form of the pinnules characteristic of the adults of 

 many species is partially or wholly absent in their young. 



Small specimens of the species of Pentametroerinidas and of the Comasteridse, 

 possibly of other families as well, possess large oral plates which persist until com- 

 paratively late in life, together with large interradials. In the Comasteridse the 

 young have the mouth and anal tube both subcentral; the mouth does not move 

 to an excentric position until a considerable size is reached; but the young of the 

 comasterids may always be differentiated from the young of species belonging to 

 other families bv the combed oral pinnules. 



