ECHINODERMA. vi 
Locomotor Organs.—The locomotor effect of the podia is due 
largely to their possession of a circular adhesive disk and a con- 
tractile wall, by means of which the animal becomes provided with 
successive points @appui; the spines may, as in “chinus, but still 
more in Spatangus, assisti nlocomotion. In Ophiuroids both podia 
and spines are greatly reduced and locomotion is effected by the 
movements of the arm-ossicles aided by powerful muscles. It has 
been stated that the common Starfish crawls at the rate of 2 or 
3 inches a minute, the Butthorn (Astropecten) moves over a foot 
and more in the same time, while a Brittle-star may writhe and 
wriggle over 5 or 6 feet in a minute. The pedicellarice—minute, 
stalked bodies with, generally, two or three jaws, which are found 
in abundance on the surface of most Starfishes and Sea-Urchins, 
though not of Brittle-stars, Holothurians, or Crinoids—may aid 
in climbing by holding on to the waving fronds of seaweed till the 
podia come into position. 
Skeleton.—No part of the body is more interesting to the collector 
of specimens than the skeleton; and it is the possession of this 
skeleton that makes it possible to form avaluable collection of dried 
examples of Echinoderms with the exception, of course, of most 
Holothurians. In these last it is either spicular, or when, as in 
some species of Psolus, it forms a set of plates, these are not con- 
tinuous over the whole of the body. In all the rest the skeleton 
consists of bars or plates which form a continuous series. 
Taking, first, the common Starfish (Asterias rubens), we find that 
the groove along the ventral side of each ray is formed by a pair 
of rod-like plates so connected together as to form a cavity; ina 
dried specimen these grooves look something like the walks (ambu- 
lacra) of a garden, and the plates are consequently known as 
ambulacral ossicles; on the outer side of each ambulacral ossicle 
there is an adambulacral ossicle; in the region of the mouth the 
ambulacrals are very large or very small in relation to the adambu- 
lacrals, and for this reason it has been proposed to form two 
divisions of the group of Starfishes (Asteroidea), which have been 
called Ambulacralia and Adambulacralia respectively. Early in 
development five of the radially placed plates which appear on the 
back move towards the ends of the arms, where they form the 
prominent terminals: the extent to which the plates of the calycinal 
area are apparent after the earliest stages of development varies 
greatly ; in the common Starfish no trace remains. The remaining 
plates are called the intermediate plates; of these those which 
border the arms are often very prominent; they form two definite 
marginal rows, and I have proposed that they should be called 
superomarginals and inferomarginals. All or any of these ossicles 
may bear spines. 
In the Ophiuroidea the arrangements of the skeleton are some- 
what more complicated: in any ordinary Brittle-star (Ophiothrix) 
the central piece of the arm (arm-bone, vertebra) consists of two 
pieces which become firmly united with one another, and have 
