570 THE ANIMAL KINGDOM. 



Formation of arm skeleton, Ludwig, Z. W. Z. xxxvi. 1882. Pedicellariae in 

 Euryalida, Id. Z.W.Z. xxxi. 1878. 



Apical system, P. H. Carpenter, Q. J. M. xxiv. 1884. 

 General anatomy, Id. Q. J. M. xxi. 1881. 



PELMATOZOA. 



Echinodermata which are fixed permanently or temporarily by an 

 aboral stem, generally jointed and containing a neuro-vascular apparatus. 

 The apical system incloses or supports the visceral mass. The oral 

 system may form a tegmen calycis or be resorbed partially or completely. 

 The water-vascular ring does not open direct to the exterior. Mouth and 

 anus on the actinal surface. 



CLASS CRINOIDEA. 



Pelmatozoa with arms more or less branched, borne upon the radials 



and in most instances furnished with pinnules. There is an ambulacral and 



anti-ambulacral nervous system. Tfie lateral extensions of the radial water- 



vasctdar -vessels form, tentacles ( = tube feet) which are disposed in groups. 



Water-tubes and water-pores lead into the coelome ; no stone-canal. 



The oral or actinal surface forms a disc, the aboral or abactinal 

 a supporting calyx. The latter consists either solely of the plates of the 

 apical system or may include also the basal joints of the arms, i. e. second 

 and third radials, &c., and the lowest pinnules. The dorso-central plate 

 retains its normal position in the few genera, all extinct, in which no stem 

 is developed, e. g. Marsupites. But in the stalked Crinoid it forms the 

 base of attachment, and is separated from the other plates of the apical 

 system by the joints of the stem *. These joints may be few, or exceedingly 

 numerous, e. g. in fossil species with stems seventy feet long, and are usually 

 provided with well-formed surfaces of union. The Comatididae break 

 away from the stem at a certain stage of growth. The topmost joint 

 of that structure then constitutes the central element of the calyx, and is 

 termed the centro-dorsal plate. Under-basals are found in some fossil 

 Palaeocrinoidea and Neocrinoidea. The basals vary from five to three 

 in number, but all living species possess five except Hyocrinus which 

 has three. In living Comatulidae except Thattmatocrinus and Atelecrinus 

 they unite into a small rosette plate which is concealed from without by 

 the centro-dorsal and radials. The first radials are five in number in all 

 Neocrinoidea ; other numbers, however, occur abnormally. They are con- 

 cealed from view by the centro-dorsal and second radials in living Comatu- 

 lidae except Thaumatocrinus and Atelecrinus. In many extinct forms 



1 Holopus is attached by an ' irregular encrusting calcareous expansion." 



