MONOGKAPH OF THE EXISTING CRICOIDS. 189 



There seems to be no difference of impoi'tance between the articulations of 

 the cirri in different types. 



ANALYSIS OF AMIILLACIIAL OSSITLES. 



The arm as a whole. — In studying the crinoid arm it must continually be 

 borne in mind that this structure is not a true appenda<je, but a local evagination 

 of the body wall, and that all of the bracliials — that is to say, the ambulacral 

 ossicles — are invariably formed in the body wall between the radials and the mouth. 



In the " cA'stid stage ■' the arms first appear as low hump-like evaginations in 

 the intersegmental (radial) angles — mere bulges in the body wall. At this stage 

 it is jjerfectly obvious that any plates formed above the radials are formed between 

 them and the mouth, on the lower slope of a swelling over the top of which is the 

 mouth. As the arms elongate this fundamental relationship becomes obscured but 

 remains unchanged. No matter how long and slender the arms may become, or 

 how many times they may be divided, they are always reducible to an evagination 

 of the body wall at the line of union between two of the original body segments 

 and between the dorsal and ventral surfaces wdiich, though becoming enormously 

 elongated and extraordinarily attenuated, still retains its primitive relation to 

 the ventral circiunoral structures by pulling out into itself ventrally loops or 

 linear extensions from their intersegmental (radial) angles, and still retains its 

 primitive relation to the skeleton forming dorsal surface by the formation of 

 a series of dorsal ossicles extending for its entire length. 



Since this is so it is evident that the connection between the dorsal and the 

 ventral surface of a crinoid in the radial regions is always along the dorsal side 

 of the arm to the tip, around the tip, and down the ventral side to the mouth, 

 and never across the base of the arm, as it would be in the case of a true appendage. 



Thus it is obvious that in reality all the brachials are formed between the 

 radials and the mouth just as strictly as if the region between the radials and 

 the mouth were a plane surface instead of being at the border line between the 

 dorsal and ventral surfaces where they are together abruptly evaginated in the 

 form of an enormously elongated process. 



The arm in detail. — The specialization of the crinoid arm has been along the 

 lines of progressively increasing length, correlated with progressively increasing 

 perfection as a uniform linear series of ossicles. 



With the formation of the primary swelling the shape of the circumoral 

 structures becomes more definitely pentagonal, and as the arm forms the angles 

 of the pentagon become drawn out into it as radial processes from the circumoral 

 rings. This radial elongation of the angles of the body is slow at first, 

 slow enough to permit the brachial ossicles to associate themselves in pairs, but 

 gradually increases in rapidity until they are formed as a linear series of exactly 

 similar elements. 



In the comatulid arm, which is the most highh' specialized arm found among 

 the crinoids, traces of the course by which the distal perfection of the arm has 

 been evolved are still to be seen. 



