28 C0RAL8 AND CORAL ISLANDS. 



The compartment between the two septa of each pair opens 

 at top into the interior of a tentacle, and thus the cavity in 

 each tentacle has its special corresponding compartment below. 

 This tentacular compartment is properly, as first recognized by 

 Prof. Verrill, the ambulacral, since each corresponds in posi- 

 tion and function to an ambulacral or tentacle-bearing section 

 in the Echinoderms and other Radiate animals. 



Although polyps are true Radiates, they have something 

 of the antero-posterior (or head-and-tail) polarity, with also the 

 right-and-left, which is eminently characteristic of the animal 

 type. This is manifested in the occurrence in some polyps of 

 a ray on the disk different in color from the general surface : 

 of one tentacle larger than the others, and sometimes peculiar 

 in color ; of two opposite septa in a calicle or polyp -cell larger 

 than the others, and sometimes meeting so as to divide the cell 

 into halves. The first of these marks the author has observed 

 in a Zoanthid, as mentioned in his Report on Zoophytes at 

 page 419, and represented on plate 30 : and the last is very 

 strongly developed in the cells of many Pocilloporae (ib. p. 523). 

 Gosse and many other authors have drawn attention to the 

 one large tentacle, and the fact that it lies in the direction of 

 the line of the mouth. Prof. H. James Clark, in his Mind in 

 Nature, states that the order in which the fleshy septa and the 

 tentacles in an Actinia are developed has direct reference to the 

 right and left sides of the body, and that there is only one 

 plane in which the body can be divided into two halves, and 

 this is that corresponding with the longer diameter of the stom- 

 ach, or the direction of the mouth. Mr. A. Agassiz has 

 shown that in Actiniae of the genus Arachnactis, the new 

 septa and tentacles are developed either side of the one chief 

 or anterior tentacle; and Prof. Verrill, that in Zoanthids, 

 they are formed principally either side of this anterior tentacle 



