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. 
Crass III. CRINOIDEA (Sea Lilies). 
CHARACTERISTICS.— The dorsal or aboral surface usually prolonged 
FI 
x 
Ie 
into a jointed stalk, by which the animal is fixed. The calyz, 
consisting of the disk and arms, in some species breaks off from 
the stalk and leads a detached existence. The jointed arms 
bear 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 Pentacrinus 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 
radial ossicles, and the last of 
these bears two brachials (Fig. 
138.—Pentacrinoid larval forms 141). These brachials form the 
of Comarda. Natural size and first of a series which form the 
axis of each of the ten arms. 
The growing point of the arm forks at short intervals, and one 
