MONOGKAPH OF THE EXISTING CEINOIDS. 



159 



Cirri have never been observed to branch, but branching is not infrequent 

 in the pinnules. Whenever it occurs it is confined to the first two segments and 

 is usually on the second (figs. 1066, 1067, pi. 15). 



TYPES OF INTERSEGMENT.iL UXIOX. 



General features. 



The intersegmental unions in the crinoids are of two types, differentiated by 

 the presence or absence of muscles. Morphologically, the difference between these 

 two types is of no great impor- 

 tance, and primarily all of the 

 unions are non-muscular. 



The non-muscular unions oc- 

 cur between the calyx plates, the 

 elements of the pairs in the divi- 

 sion series and the first two 

 brachial pairs, and at intervals 

 throughout the arms. 



The muscular articulations 

 include the majority of the 

 unions between the brachials, and 

 the articulations at either end 

 of the pairs of ossicles forming 

 the first two brachial pairs and 

 the division series, including the 

 articulation on the distal end of 

 the radials. 



All of the plates composing the 

 external skeleton of the crinoids. 

 perisomic plates, pinnulars, bra- 

 chials, calyx plates, columnals, 

 and cirrals originate in exactly 

 the same way in a uniform layer 

 of mesodermic tissue. 



Where they are widely sep- 

 arated, as in the case of poorly 215 

 developed side and covering &$ 

 plates, there is no modification 

 of the tissues between them. 



If they are near together 

 but very rudimentary they are 

 commonly more or less united by strands of connective tissue. Covering plates are 

 commonly united to the corresponding side plates in this way. 



When they are very close together they are firmly united by parallel liga- 

 ments, the ends of which form loops within their substance. The apposed sur- 

 faces of ossicles thus united may be smooth, as in the case of the unions between 



Fiu. 215. LATEBAL VIEW OF TYPE SPECIMEN off COSMIOMETKA 

 ASTEK. 



