THE CRINOIDEA 99 



tained to be permanently attached to extraneous bodies, whilst 

 others appear to have been capable of locomotion." So little is 

 amiss with this description, that we need do no more than trans- 

 late it into modern terminology, as follows : 



A normal Crinoid (Fig. III.) consists of a " crown " (corona) 

 attached by its dorsal (i.e. aboral) extremity to a " stem " (columna), 

 which is fixed to the sea-floor or to some solid body by a " root " 

 (radix). The crown consists of a theca (or calyx, in the sense 

 of Wachsmuth & Springer) containing the viscera, and of 5 

 " arms " (brachia), which may be more or less branched. That 

 part of the theca below the origins of the free arms is called the 

 " dorsal cup " (or shortly " cup ") ; that part above the origins of 

 the free arms, i.e. the oral surface, is called the legmen (sometimes 

 " disc," sometimes "vault," between which a distinction erroneously 

 used to be imagined). The skeletal and many of the other 

 systems have a radiate arrangement, of which 5 is the dominant 



M0 



BASE - 



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Fio. IV. 

 Imaginary analyses of the structure of the dorsal cup in two simple types of Crinoid. 



number. Thus the whole animal can be divided into 5 cor- 

 responding and almost symmetrical sections, " pentameres," by 

 5 imaginary "perradial planes," starting from the vertical 

 dorso-ventral axis and passing through the origins of the arms. 

 The skeletal elements are either perradial or interradial in 

 position. 



The Dorsal Cup in its simplest form is composed of 2 or 3 

 circlets of 5 plates, those in one circlet alternating with the 5 in 

 the adjacent circlet (Fig. IV.). Of these the most important are those 

 that support the brachia, and to them the term radialia (RR) is 

 restricted. The interradial plates below these are called basalia 

 (BB), since in many crinoids they form the base and rest on the 

 stern. In some crinoids a circlet of perradial infrabasalia (IBB) 

 occurs beneath the BB (which latter are then called parabasalia 

 by some writers). The former type of base is called " monocyclic "; 

 the latter " dicyclic." 



The Tegmen in its simplest form is likewise composed of 5 



