376 COMPARATIVE ANATOMY CHAP. 



(d) The Manner of Connection between the Skeletal Pieces. 1 



Under this head we have to consider the method of connection between the 

 ossicles of the arms and of the pinnulse, between the plates of the apical capsule, and 

 between the coltunnals. 



Perhaps the clearest view of the great diversity which prevails in this matter is 

 obtained by assuming that the plates composing an echinoderm skeleton develop 

 in a stroma of connective tissue fibrils ; all the plates might thus be supposed to have 

 been originally but loosely united by such fibrils. 



This condition persists in what is known as the loose suture. The ossicles of the 

 pinnules in many living Crinoids are united in this way. 



From this loose suture we can obtain all the man}- variations which are found 

 either in the direction of greater rigidity or of greater flexibility. 



In the former case we have : 



1. The close suture, also known as synostosis, in which the connecting fibres 

 are short, and " the joints closely and immovably fitted together, though they can be 

 separated by the action of alkalies," e.g. the radials of Antedon. 



2. The syzygy, which is a special case of close suture, viz. that in which, if 

 pinnules or cirri are carried by the ossicles, the lower one loses its pinnule or cirrus. 



The two components of a syzygy are termed epizygal and hypozygal. 



3. Anchylosis, in which the two plates or ossicles are immovably cemented 

 together by an unbroken deposit of calcareous substance, which, however, is less 

 solid than that of the plates themselves (e.g. the basals of Bathycrinus and the 

 radials of Rhizocrinus). 



On the other hand we have the modifications in the direction of greater flexibility 

 which lead up gradually to the development of true muscles, the original undiffer- 

 entiated fibrils becoming muscularly contractile. Such muscular articulations may 

 indeed be very highly specialised, with interlocking ridges and teeth on the opposed 

 facets of the ossicles. 



As all these different modifications pass into one another by imperceptible stages, 

 it is not always easy to say whether in any special case we have to do with muscular 

 articulation or with a less specialised form of connection. It now seems probable 

 that many of the fibrous connections which were at one time thought to lie only 

 elastic fibres are really muscles. 



For instance in the arms of living Crinoids, a pair of muscles on the oral side are 

 counteracted by a pair of bundles of " elastic " fibres on the dorsal side. The action of 

 the muscles in contracting rolls the arms up orally, and on the muscles relaxing, the 

 " elastic " fibres expand the arms again. Now it is clear that if these fibres were 

 thus simply elastic, Crinoids would die with their crown of tentacles expanded, 

 whereas the reverse is the case. 



Again, the cirri are actively movable, often (e.g. in Pentacrinus) more so than 

 the arms, although no muscular articulations occur, in them. 



From these facts it is rightly argued that the fibrils in these cases, though differ- 

 ing histologically from the true muscles, are yet to some extent muscular. 



That all these various connections are in reality derivations from some common 

 primitive form of connection, we gather from the fact that in the stems of Crinoids 

 we may have anchylosis, close suture, syzygy, loose suture, and true muscular 

 articulation. 



1 This passage was rewritten in accordance with the ten our of Mr. Bather's criticism 

 in Natural Science, vol. vi. pp. 420, 421. TR. 



