92 MARINE ANIMALS OF MASSACHUSETTS BAY. 



the Star-fislies and Sea-urchins carry it upward, and the Holothu- 

 rians, moving with the mouth forward, have the dorsal system 

 at the opposite end of the body. Whatever the natural attitude 

 of the animal, however, and the consequent position of the dorsal 

 region, it exists alike in all the five orders, though it has not the 

 same extent and importance in each. But in all it is made up of 

 similar parts, bears the same relation to the rest of the body, has 

 the same share in the general economy of the animal. And 

 though when we compare the spreading back of a Star -fish with 

 the small area on the top of a Sea-urchin, where all the zones 

 unite, we may not at once see the correspondence between them, 

 yet a careful comparison of all their structural details shows that 

 they are both built with the same elements and represent the 

 same region, though it is stretched to the utmost in the one case, 

 and greatly contracted in the other. 



This being true of the dorsal system, let us look at another 

 equally important structural feature in this class. All Echino- 

 derms have locomotive organs peculiar to themselves, a kind 

 of suckers which may be more or less numerous, larger or 

 smaller, in different species, but are always appendages of the 

 same character. These are variously distributed over the body, 

 but always with a certain regularity occupying definite spaces, 

 shown by investigation to be homologous in all. For instance, 

 the rays of the Star-fish correspond in every detail on their under 

 side, along which the locomotive suckers run, with the zones on 

 the Sea-urchin, from end to end of which the suckers are ar- 

 ranged ; and the same is equally true of the distribution of the 

 suckers on the Holothurians, Ophiurans, and Crinoids, though, 

 as most persons are less familiar with these orders than with 

 the other two, it might not be so easy to point out the coin- 

 cidence to our readers. These suckers are called the ambu- 

 lacra, the lines along which they run are called the ambulacral 

 rows or zones, while the system of locomotion as a whole is 

 known as the ambulacral system. Since these organs are thus 

 regularly distributed over the body in distinct zones or rows, it 

 follows that the latter must be divided by intervening spaces. 

 These intervals are called the interambulacral spaces ; but while 

 in some orders they are occupied by larger plates and prominent 



