-tO Clark — Origin of Crinoidal Muscular Articulations. 



Art. II. — The Origin of the Crinoidal Muscular Artic- 

 ulations ; by Austin Hobart Clark. 



The peculiarly complicated type of muscular articulation by 

 which the post-radial ossicles of the crinoids are joined together 

 is generally supposed to have been derived from the so-called 

 loose suture, the connective tissue of which has gradually 

 become differentiated into two different types of ligaments 

 and also into true muscle. The steps by which this process 

 has come about have never been satisfactorily shown. 



While there appears to be in the crinoids a direct continuity 

 between the connective tissue through various types of liga- 

 mentous attachment to true muscle, yet it does not seem 

 probable that the differentiation of the connective tissue 

 between two adjacent ossicles could ever have progressed so far 

 as to produce the conditions found in the crinoids, where two 

 bundles of highly specialized muscle fibers occur, histologically, 

 as well as in their location, sharply differentiated from the 

 ligaments. 



A comparison of the crinoids with the two most nearly related 

 recent classes, the Echinoidea and the Holothuroidea (Bohad- 

 schoidea), offers, however, an easy solution of the problem of 

 the origin of the complex crinoidal muscular articulations. 



In the Echinoidea the plates are united more or less closely 

 by connective tissue, just as are the interradials and the 

 secondary perisomic plates generally in the recent crinoids, and 

 this was probably the original mode of union for the primary 

 ambulacral ossicles of the crinoids as well, Now in all the 

 heteroradiate Echinodermata (which include the Pelmatozoa, 

 Echinoidea, and Holothuroidea) wherever the ambulacral ossi- 

 cles or body wall are at all flexible, as occurs in the crinoids, in 

 the holothurians, and in the echinothurids among the echinoids, 

 each ambulacrum possesses a pair of longitudinal muscles one 

 of which runs along either outer border, and may extend itself 

 more or less inward. No definite homology has ever been 

 proved between these five longitudinal ambulacral muscle pairs 

 which are so constant in all the recent heteroradiate Echino- 

 dermata possessing the possibility of ambulacral motion ; but 

 from the uniform presence and location of these muscles it 

 seems most probable that such homology actually exists. 



Now if the ancestral crinoids possessed a longitudinal muscle 

 along each border of the ambulacral series, as we must infer 

 from analogy with the echinothurids and the holothurians, 

 their closest recent relatives, we may assume an ambulacral 

 structure something like that of the former — a series of 



