370 COMPARATIVE ANATOMY CHAP. 



radius, immediately behind the centre of the tegmen, this dome is produced still 

 further into a long tube similarly plated ; this is the anal tube, on the tip of which 

 lies the anus. 



The At-ticulata, so far as the perisomatic plates of the calyx are concerned, 

 agree with the Camcrata in that the ossicles of the arms are sometimes incorporated 

 into the dorsal cup as far as to their second or third divisions (Fig. 322), the 

 primary, secondary, and often also the tertiary brachials becoming fixed plates 

 of the dorsal cup. The number of the brachials in each arm and its branchings 

 varies. Three primary brachials are often found in each radius. But these fixed 

 brachials are not, as in the Camernta, rigidly connected inter sc and with the radials, 

 but are articulated. The spaces on the dorsal cup between the radii and between 

 their branchings are filled either with quite small, loose, and irregular calcareous 

 corpuscles or scales, or with small, definitely arranged plates (interradials, inter- 

 distichals, etc.). In the posterior interradius there are often, in addition, special anal 

 plates frequently asymmetrically arranged. 



The tegmen calycis of one .species of the genus Taxocrinus is" w r ell known. The 

 radii and their branchings are bulged out while the interradii are depressed. From 

 the central mouth, which is open and surrounded by five orals, the five ambulacra! 

 furrows run out, dividing dichotomously in correspondence with the branching of 

 the arms. Each ambulacral furrow has a fioor of two longitudinal rows of sub- 

 ambulacral plates, is bordered by lateral plates, and closed in by two longitudinal 

 rows of covering plates. The covering plates in the two rows are alternately 

 arranged, their interlocking forming a zigzag line, and it is very probable that they 

 were movable, i.e. that they could be raised and depressed. The interambulacral 

 regions contain a large number of small, loose, irregular plates. In the posterior 

 interradius, at the edge of the tegmen, there is a plated process (anal tube ?). 

 For Thaumatocrinus, see "The Systematic Review" (p. 309). 



(/>) The Braehial Skeleton. 



The calyx of the Crinoidea carries at its edge (on the boundary 

 between the tegmen calycis and the dorsal cup) five arms, which 

 are rarely simple, but usually branched, and in the living animal are 

 beautifully extended. The arms can be made by stimulation to 

 fold together over the tegmen. They are found in this position 

 also in dead animals, and therefore almost always in fossilised 

 individuals. 



The arms, which contain important inner organs, are supported 

 by a special brachial skeleton. This consists of consecutive calcareous 

 pieces, the brachials, which are either firmly connected or articulated 

 with one another. The brachials are deepened on their oral side, 

 that which is directed upwards when the arms are spread out, to form 

 a more or less deep longitudinal groove along the arms and all their 

 branches ; this is the ambulaeral groove. In the base of this groove 

 lie the most important inner organs of the arms (radial canals, water 

 vessels, outgrowths of the body cavity, etc.). The soft integument 

 which covers these organs, and stretches over the ambulacral grooves 

 of the brachial skeleton is in its turn depressed to form a channel. 

 These integumental channels, which accurately correspond with the 

 ambulacral grooves of the skeleton, are called food grooves. At the 



