24 BRITISH FOSSIL CORALS. 
concave track, and the mouth projects in the form of a more or less oblong truncated 
cone. There are from sixteen to eighteen internal folds, faintly shown, however, on the 
rim around the mouth. 
“The tentacules are of the same number as the septa whose summits they envelope, and 
there are always from thirty-two to thirty-six. They are evidently equal in size and in 
length. Their length is nearly equal to that of the diameter of the corallite. They 
are elongated, swelling a little above their insertion, and then becoming very slender as far 
as their free extremity, which is terminated by a small knob-shaped enlargement. 
“The polypes can contract to various extents. Several very characteristic movements 
may be noticed, however. A slight agitation of the surrounding water or the contact with 
small particles, suffices to cause a shortening of some or all of the tentacules, although the 
disc does not alter its shape or position. 
«When the exciting cause acts more decidedly and continuously, the shortening of the 
tentacules increases, the disc retreats, and the protractile mouth elongates in advance of the 
calice. This state of things is very usual in disturbed or decomposing water. If the 
animal itself is shaken or is touched, it retracts its disc into the calicular fossa, and 
nothing is to be seen of the soft parts but some small elevations corresponding with 
the tentacules. Finally, a violent shock or a prolonged irritation produces so complete a 
retractation that the tentacules disappear completely, and the white colour of the septa is 
seen. The calice looks as if it were dried, and there is only a light brown tissue in the 
interseptal loculi. In this last case the water which usually distends the tissues has been 
cradually expelled, and they are so reduced in volume that they are readily withdrawn 
into the interseptal and columellar spaces. 
«The disc and the tentacules are of a transparent brown colour, and when the sun 
shines, a brilliant green tint may be seen within the tentacules.. ‘This coloration evidently 
depends in some instances upon the light. But it is necessary to remark, that the 
primary and secondary tentacules and those of the third cycle which are flanked by 
quaternary are those which show this green tint in their insides. The peculiarities of these 
tentacules coincide with the presence of pali, which are situated beneath and within them. 
“When the mouth opens, as it often does when the polype is semi-retracted, the 
papillze of the columella are visible. The stomach is very short, and is almost reduced to 
arim, which is confounded with the lips. 
«The tentacules are not smooth, but are covered with a multitude of small wart-shaped 
prominences, of a transparent white colour ; they are equal in size, and measure a tenth 
of a millimétre in width. The terminal bulb presents a narrow central canal, which 
communicates both with the tentacular cavity and with the external medium. ‘The 
three layers of tissue which constitute the tentacules have the same general characters 
as in the Actinia, but the four layers of the tegumentary covering are not to be 
detected. 
“1, The first envelope is quite transparent, and is composed principally of nematocysts of 
