292 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



The region of the anus in the regular echinoids is 

 primitively occupied by a single large plate, the 

 dorso-ceiitral, and, in the study of the morphology of 

 the apical area of Echinoderms, it is necessary to always 

 bear in mind (I) the dorso-central, (2) the radial, and 

 (3) the basal plates, which are interradial in position. 



Within the test of the Echinus are five calcareous 

 arches (auriculae) which afford attachment to the 

 " Lantern of Aristotle "; these auricles are, when 

 present, radial in position in all Echinoids, except the 

 Cidarida, where they are interradial. 



In the adult Crinoid we distinguish a cup, or 

 calyx, which may, as in Pentacrinus, be permanently 

 fixed by a stalk, or, as in Antedon, be stalked in the 

 larval stages only. In both cases the calyx gives off 

 a number of arms, which consist of numerous small 

 calcareous joints, and have jointed appendages, the 

 pinnules, attached to them. However numerous 

 these arms may be, and there may be almost one 

 hundred, we find that, as we trace them back to the 

 calyx, they form branches of one or other of its five 

 rays. These rays ordinarily consist (the common 

 Antedon of our own seas is an excellent example) of 

 three radial joints ; all these joints unite to a common 

 central piece, which is known as the centre-dorsal. 

 In the stalked forms this centro-dorsal is placed at the 

 top of the stalk, which consists of a large number of 

 small ossicles, and is at various points along its length 

 provided with jointed five-rayed outgrowths (cirri), 

 which have a claw at their free end. In Antedon and 

 others, where the stalk is lost in adult life, the cirri, 

 which vary a good deal in number, and are sometimes, 

 after a certain age, completely lost, are directly attached 

 to the centro-dorsal itself. 



We apply the term centro-dorsal to the central 

 plate of the crinoidal calyx to distinguish it from the 

 dorso-central of the typical unaltered apical area of 



