278 PHYLUM ECHINODERMATA. 



the secundaxil ; and so on, tertibrachs, quartibrachs with tertaxil, 

 quartaxil, etc., for the axillary pieces. 



In Crinoids with a simple larviform calyx, viz. the Larviformia, 

 some Fistulata, Marsupites, Hyocrinus, etc., the whole of the arm 

 is free, no part of it assisting in forming the calyx. In this case 

 all the brachials are said to be free. But in most Crinoids a 

 certain portion of the lower part of the arm is incorporated in 

 the calyx, which is thereby enlarged. In this case those 

 brachials which are incorporated in the calyx are called fixed 

 brachials. As a general rule when the calyx is thus enlarged 

 a number of interradial plates are found in the wall of the calyx 

 between the fixed brachials : these may be called interprimi- 

 brachs, inter secundibrachs, etc., according to their position be- 

 tween primibrachs, secundibrachs, etc. (Fig. 203). 



In the Larviformia and Fistulata there are no interradial plates in the 

 calyx excepting the anal plate or plates in the posterior interradius (Fig. 

 201). In Thaumatocrinus and the Rhodocrinidae there are five interradial s 

 regularly disposed around the calyx between the radials, and touching the 

 basals ; in Thaumatocrinus the anal interradius is marked by a short external 

 jointed process. In the Camerata and Flexibilia, calyx interradials are 

 generally present between the brachials (Figs. 193, 203), but not between 

 the radials except in the Rhodocrinidae and except in the anal interradius, 

 in which anal plates can often be made out. In the Articulata, calyx 

 interradials are generally absent ; they are present in some Apiocrinidae 

 between the primibrachs , but are absent in the Encrinidae (Fig. 205), 

 Bourgueticrinidae, Pentacrinidae, and Comatulidae. In the Encrinidae 

 no part of the arm is incorporated in the calyx, and in the last three 

 families mentioned the body-wall which connects together those portions 

 of the arms which are incorporated into the calyx resemble the calyx 

 cover in either being without definite plates, or in having loosely arranged 

 irregular plates. It thus appears that the radials form an uninterrupted 

 circle in Articulata. In most other Crinoids they are interrupted in the 

 anal interradius only, excepting in Thaumatocrinus and the Rhodo- 

 crinidae, in which they are interrupted in every interradius. 



The brachial plates at the base of the arms are always in a single row, 

 and in most Crinoids this arrangement is continued throughout the arms : 

 this is the uniserial arrangement. But in some of the later palaeozoic 

 forms the distal plates may be wedge-shaped, the broad end of the 

 wedge being alternately on the right and left side of the arm : this is the 

 alternate arrangement. Finally the plates may be in two series, the 

 contact surfaces interlocking so as to give rise to a zigzag line ; this is the 

 biserial arrangement and often follows the alternate arrangement as the 

 end of the arm is approached. 



The pinnules are modified arm branches. They alternate on 

 the two sides of the arm and are often closely crowded. They 

 contain all the organs found in the arms, are jointed like the 



