236 



ZOOLOGY 



northern European seas. Like many other members of the 

 class, it is brilliantly coloured. The different specimens of the 

 same species exhibit a surprising amount of variation both in 

 their colour and markings. 



Class III. CRINOIDEA (Sea Lilies). 



Characteristics. — The dorsal or aboral surface usually prolonged 

 into a jointed stalk, hy which the animal is fixed. The calyx, 

 consisting of the dish and arms, in some species breaks off from 

 the stalk and leads a detached existence. The jointed arms 

 hear lateral pinnules. The tube-feet take the form of ten- 

 tacles arranged in groups on the disk, arms, and pinnules. 

 No madreporic plates exist, but certain holes lead from the 

 water-vascular system into a ramifying system of vessels, 

 whence others open to the exterior. 



The skeleton of the Crinoids is composed of a number of 



ossicles with a very definite 

 arrangement. The topmost seg- 

 ment of the jointed stalk is 

 termed the centro-dorsal plate ; 

 in the Comatulidae, which lose 

 their stalk when adult, this 

 persists as the central aboral 

 plate, and bears several whorls 

 of cirrhi which have a root-like 

 appearance. The stalked forms, 

 such as Penfacrinus caput Med- 

 usae, also have numerous cirrhi, 

 arranged in whorls on their stalks. 

 From the centro-dorsal piece five 

 radial plates radiate; these are 

 continued by second and third 

 V' radial ossicles, and the last of 



N these bears two brachials (Fig. 



Fia. 138.— Pentacrinoid larval fonns 141). These brachials form the 



of Coinatula. Natural size and fy.^^ „£ g, series which form the 

 magnified. 



axis of each of the ten arms. 

 The growing point of the arm forks at short intervals, and one 



