OF CENTRAL CANADA PART IV. 



237 



Echinodermata. The more common existing forms comprise " star- 

 fishes," " brittle stars," " sea-urchins," and holothurians. Peduncu- 

 ]ated forms were especially characteristic of the earlier periods of the 

 Earth's history, but are now comparatively rare, or, as regards the 

 greater part, entirely extinct. 



The Echinoderms are distributed under the following classes : 



1. Crinoidea. 5. Opliiur oidea. 



2. Cystidea. 6. Aster oidea. 



3. Blastoidea. 7. Echinoidea. 



4: Edrioasterida. 8. Holothur oidea. 



CRINOIDEA. 



This class comprises the various so-called " sea-lilies," now nearly 

 extinct. The Oinoids are attached, typically, to the sea-floor by a 

 comparatively long, flexible stem ; but some become free in the adult 

 condition. They consist essentially of three parts : the body or 

 "calyx, the "arms" or tentacles, and the stalk or stem, as indicated 

 in the annexed sketch, figure 150. The body, oval 

 or cyathiform in shape, is protected externally by 

 a number of calcareous plates, meeting, and in some 

 cases partially interlocking, at their edges. These 

 plates comprise : (1) a series of " basals," usually 

 three or five in number, immediately above the 

 stem, but often forming two horizontal rows, known 

 respectively as lower and upper basals ; (2) a 

 series of " radials " often in more than one zone ; 

 (3) a series of " inter-radials," more or less nume- 

 rous, but sometimes absent; and (4) a series of 

 "brachials," from which the arms or tentacles 

 immediately spring, (see figure 151). The upper 

 part of the calyx is covered in many genera by 

 numerous small, more or less irregularly arranged 

 plates, and is termed the " vault " or " roof," but 



FIG. 150. 



this in most of the more modern forms is simply coriaceous, or desti- 

 tute of plates, properly so called. The calyx has a central opening, 



