198 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The addition of brachials distally after maturity is so slow that the general pro- 

 portions of the arm length to the other dimensions is not appreciably altered. 



For purposes of description a crinoid may be conveniently considered as made 

 up of calcareous ossicles and "soft parts." To be exact, a crinoid should be consid- 

 ered as having no "hard parts," for the inorganic elements are not deposited in a 

 specially differentiated and localized matrix, but make then- appearance anywhere. 

 It is not always easy to decide whether certain organs should be included under the 

 head of calcareous or of noncalcareous components of the crinoid whole. Such 

 organs I have associated with others of an equivalent systematic value rather than 

 with those of similar morphological significance. 



In a very large 10-armed comatulid in which side and covering plates are 

 developed there are visible externally about 600,000 distinct skeletal elements, 

 each of which arises from a separate center of ossification; of these about 87,000 

 belong to the primary and about 513,000 to the secondary or perisomic skeletal 

 series. In a large comasterid with no side and covering plates developed there may 

 be as many as 700,000 primary skeletal elements visible, while in the very smaU 

 antedonids the number probably never falls below 10,000. The greatest of these 

 figures is insignificant, however, when compared with the number of ossicles in the 

 larger pentacrinites where, in the recent species, nearly 1\ millions are found. 

 These figures, large as they are, must be approximately doubled when the internal 

 skeleton is taken into consideration. 



Column. 



Except for the short period during which the animals are free-swimming cili- 

 ated bilaterally symmetrical larvae, the young of all recent comatulids so far as 

 known are, until a considerable size is reached, attached to the sea floor or to other 

 organisms by a slender column of essentially the same type as that found in the 

 species of the family Bourgueticrinid83 (figs. 532, 533, pi. 3, and 543, pi. 4). 



This column varies very greatly in its proportionate length and in the relative 

 proportions and number of its component segments, as will be explained in detail 

 in the section dealing with the Pentacrinoid Larvae. 



The column of the crinoids as a whole is the equivalent, collectively as well as 

 in each individual segment, of the central or suranal plate of the echinoids in which 

 such a plate is developed (fig. 71, p. 127), and of all the small plates of the peri- 

 proctal area taken together in the echinoids in which no central or suranal plate 

 occurs (fig. 72, p. 127) ; speaking more broadly the crinoid column is the equivalent 

 of a crustacean cephalothoracic appendage, or a group of five such appendages. 



The central or suranal plate of the echinoids is not, like the plates of the coronal 

 ring, an element of fundamental phylogenetical significance; but it represents the 

 resultant from the coalition of numerous small plates and spicules of the periproctal 

 area, a coalition which has taken place within the class at a comparatively late 

 phylogenetic stage and does not occur in the earlier forms. 



The central plate of the echinoids within that group is purely a secondary 

 plate, confined to the later and more specialized types, in which it is of somewhat 

 irregular occurrence and of equally irregular morphological value. 



