ECHIXODERilA. / 



Locomotor Onjaas. — 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 d'appui; the spines may, as in Echinus, but still 

 more in Sjxitaat/us, assisti n locomotion. 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 (Astropectai) 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 pedicellariae- — 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. — ISo 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 a valuable 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 ; in a 

 dried specimen these grooves look something like the walks (ambu- 

 lacra) of a garden, and the plates are consequently known as 

 ambulacra! 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 Ophii:roidea the arrangements of the skeleton are some- 

 what more complicated: in any ordinary Brittle-star (Opliiothrix) 

 the central piece of the arm (arm-bone, vertebra) consists of two 

 pieces which become firmly united with one another, and have 



