viii ECHINODERMATA MORPHOLOGY OF SKELETON 365 



The Canaliculata, like the more recent Inadunata (Lias to present time), an- 

 distinguished by the regular radial structure of the dorsal cup, in which interradials 

 only exceptionally occur, and special plates in the posterior interradius (anals) never 

 occur. Very often (Apiocrinus, Rhizocrimis, Antedonidcv) two or more brachial 

 plates following the radials of the calyx are incorporated as "fixed brachials" into 

 the dorsal cup. 



In connection with the tegmen calycis, it must be noted that among the Canali- 

 culata, orals appear in the adult only in Rhizocrinus. As a rule, the tegmen calycis 

 is plated in the interambulacral regions with numerous loosely connected skeletal 

 pieces, which vary in size according to the species and genus. These small plates 

 are perforated by pores. This skeletal covering is not infrequently continued on to 



fa fa, 



FIG. 316. Tegmen calycis of Metacrinus angu- 

 latus, P. H. Carp, (after P. H. Carpenter), o, Mouth; 

 In-, anus ; p, pinnulse (both broken off) ; ta, anal tube, 

 near which there is a second abnormal tube to] ; cpa, 

 covering plates of the ambulacral furrows. 



FIG. 317. Actinometra strota, P. H. 

 Carp, (after P. H. Carpenter). Tegmen 

 calycis. o, Mouth ; an, anus ; a/a, food 

 grooves of the arms ; afd, the same of the 

 tegmen ; pi, two pinuulce, which take the 

 place of one of the two posterior arms. 



the bases of the arms, and occasionally runs out between these apically in such a 

 manner as to be visible in the interradii of the dorsal cup. 



The ambulacral furrows of the tegmen calycis are rarely open, but usually covered 

 with covering plates and often bordered by lateral plates (Fig. 316). Occasionally 

 the mouth also may be covered with perisomatic plates, but it is usually open. 



The anal tube in the posterior interradius varies in size and in its position within 

 this interradial area. Its plating agrees with that of the interambulacral area on 

 which it is found. 



The interambulacral areas may also be naked, i.e. covered with integument 

 containing only very small calcareous corpuscles. 



Actinometra is the only recent Crinoid in which the mouth is found 

 placed quite excentrically (anteriorly) on the tegmen, and the anus, 

 which lies in the enlarged posterior interradial area, comes to lie 



