582 INVERTEBRATE MORPHOLOGY. 



the ambulacra or areas occupied by perforated plates are 

 termed petaloid. In Echinarachnius, for instance, the plates 

 after having reached their greatest width retain it to their 

 abrupt termination (Fig. 267), the petals being then termed 

 open, but in other forms, e.g. Mellita, they contract again 

 peripherally, in which case the ambulacra are said to be 

 closed. 



The pores belonging to each pair are generally united by 

 a groove, and are termed yoked pores, and in Mellita, for 

 example, in addition to the pair of yoked pores on each plate 

 there is a third one situated near the middle line of the am- 

 bulacrum. The tube-feet which project from the yoked pores 

 are frequently pinnate in form, while those emitted through 

 the single pores are simple and tentaclelike. The spines are 

 generally very small, though those of the oral surface serve 

 for locomotion. 



In Mellita, towards the periphery of the test, the im per- 

 forate auibulacral plates of the radii A, B, C, and E do not 

 meet, leaving elongated holes passing through the test, and 

 the same thing also occurs with the plates in the interradius 

 AB, so that altogether five such holes exist. In other forms, 

 instead of holes, notches occur at the margin of the test, and 

 other interambulacra than that in which the hole occurs in 

 Mellita may be affected. Calcareous columns extend from the 

 oral to the aboral surfaces of the test, being especially abun- 

 dant towards the periphery, and calcareous plates uniting the 

 two surfaces occur on either side of each ambulacrum. An 

 Aristotle's lantern is present, but the alveoli are usually 

 broader than long. 



3. Order Petalosticha. 



In this order, as its name indicates, the ambulacra are 

 usually petaloid, and the bilaterality indicated in the Clypeas- 

 troids is more pronounced, since neither the mouth nor the 

 anus retains its original position at the centre of the oral or 

 apical surface. The anus lies in the posterior interradius 

 A B, while the mouth has moved forwards to a greater or less 

 extent along the radius D. The test is oval or, frequently, 



