vin ECHINODERMATA MORPHOLOGY OF SKELETON 363 



in the dorsal cup and in the tegmen. They are called interradials. 

 In the tegmen they develop in the zone between the orals and the edge 

 of the calyx, and belong to the interambulacral system of plates. 

 Usually, only the interradials of the dorsal cup are so called, although 

 they are not infrequently continued, between the bases of the arms, 

 into the interradial system of plates of the tegmen without any sharp 

 boundary. 



c. The proximal portions of the arms may, to a greater or lesser 

 extent (to their first, second, etc. divisions), be taken into the calyx, 

 in which case the skeletal segments of the arms (brachials) become 

 perisomatic plates of the dorsal cup, and are known as fixed braehials 

 (primary, secondary, etc., formerly called radials of 1st, 2nd, etc. 

 orders). (For the meaning of these names, see below, the section on 

 the brachial skeleton, p. 370.) 



d. Just as interradials may appear in the dorsal cup between the 

 five radials and the fixed branchials of the five radii, so the branches 

 of each arm incorporated into the calyx may themselves be connected 

 by intercalated plates. Those which lie between brachials of the 

 second order are then called interdistiehals or interseeundibraehs, 

 those between brachials of the third order (after the second forking) 

 interpalmars or intertertibraehs, etc. 



When more than five free arms rise from the edge of the calyx, i.e. 

 when some length of the arms and their branches is incorporated into 

 the calyx, the food-grooves running over the tegmen from the mouth 

 to the periphery divide dichotomously in such a way that the number 

 of grooves ultimately corresponds with that of the free arms. The 

 regions between the branches of the five primary radial food-grooves 

 are as a rule also plated with small interambulacral pieces. 



e. The food-grooves running over the tegmen from the mouth 

 to the bases of the arms very often have a skeleton of their own, 

 which may be continued into the ambulacral furrows of the arms and 

 their branches. This ambulaeral skeleton may consist of lateral 

 plates (which border the furrow laterally) or of covering 1 plates (which 

 cover the furrows, changing them into passages or tunnels), or of both 

 these sorts of plates. Subambulacral plates may also occur. 



Special Remarks on the Perisomatic Skeleton of the Crinoid Calyx. 



In the Iiiadunata larviformia (Type : Haplocrinus) there is no perisomatic 

 skeleton of the calyx. This latter consists exclusively of the plates of the apical 

 and oral systems (five basals, five radials, three of which are transversely divided, 

 and five orals). 



The first perisomatic plate of the calyx occurs in related forms, interradially, in 

 the radial circle, and rests upon the posterior basal ; it is the anal. 



As a type of the Inadunata fistulata, we shall first select Cyathocrinus. In the 

 dorsal cup only one perisomatic plate appears, which is found resting on the 

 posterior basal, between the two posterior radials (Fig. 289, p. 329). The apical 

 capsule thus altogether resembles that of the Larviformia. The tegmen calycis, 

 on the contrary, shows an entirely different condition, which, however, may vary 



