28 CORALS 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 a@mbulacral, 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 
aray 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 Zodphytes at 
page 419, and represented on plate 30: and the last is very 
strongly developed in the cells of many Pocillopore (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 Actinie 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 
