PORITES. 23 
The polyps secrete a great deal of slime (¢f. Duerden, 1. c. p. 415), one function of which, as 
I have been informed by Mr. Thurston, is to protect colonies from sun and air when exposed 
by the tide. It is secreted in large quantities, and may hang down in streamers from specimens 
picked up from the reef. 
The colouring, so far as the Great Barrier Reef specimens are concerned, is not very 
brilliant, according to Mr. Saville-Kent. Light ochres, dark and golden or mustard yellows, and 
browns, are the prevailing colours among the branching forms, while massive forms are often a 
delicate pink, light or bright lilac, more rarely a pale yellow. The oval disc and bases of the 
tentacles are usually the same colour as the corallum, but the tips are greenish grey. Duerden, 
ou the other hand, speaks of the rich yellows, greens and blues of the Porites of the West Indies. 
The colouring is said to be due to three sources : (1) regular pigment cells ; (2) zooxanthelle in 
the tissue of the polyps; and (3) alge which bore into the skeleton. The prevailing yellow-brown 
is due to the presence of the yellow pigment-cells and the zooxanthelle. The colours given to 
the skeleton by the boring alge were found by Duerden to be bright green or pink, due, 
apparently, to the colours of the alge, green or red. The dried skeletons become gradually 
bleached. 
Dr. Duerden discovered that coloration depends upon the light; under sides or shaded 
stocks being nearly always devoid of colour. 
Mr. Saville-Kent figured the tentacles with large swollen knobs at their tips; this 
power of swelling the tips of the ordinary digitiform tentacles is probably general. Duerden 
shows a case (I. ¢. fig. 32a) with much smaller swellings. 
The digitiform tentacles are frequently uniform in size, but sometimes a_ bilateral 
symmetry is visible, in that the two tentacles in the longer oral axis are the largest, while 
those next them on each side are the smallest. This, found also in Madrepora, is said by 
Dr. Duerden (1. c. p. 427) to be a retention of a larval condition in the development of 
the tentacles of certain Actiniaria, and is associated with a primitive condition of the internal 
mesenteries. 
The primitive character of the Madreporarian polyp is shown in the fact that there is no 
siphonoglyph. In Porites the deep longitudinal folds, which necessarily arise on the contraction 
of the stomodzal walls, form, according to Duerden, symmetrical figures. 
Mesenterial filaments are feebly developed, being confined to two or three of the pairs of 
mesenteries (see Duerden, 1. c. p. 475). 
