10 



ON THE STRUCTURE OF THE HALCYONOID POLYPI. 



AFTER removing the so-called Hydroid Polypi from the class of 

 Polypi, to place them among Medusce, and the Bryozoa being referred 

 to their true natural position among Mollusca, the class of Polypi 

 contains only two natural groups of animals, the Actinoid Polypi, 

 and the Halcyonoid Polypi. 



Having, on a former occasion, given an account of my investiga- 

 tions, of the former group, I will now proceed to make some remarks 

 upon the Halcyonoid Polypi of the United States, which I have 

 had an opportunity to examine. They belong to three different 



genera. 



Renilla. 



One of these genera is the well known Renilla reniformis, which 

 is extremely abundant on the Southern shores of the United States. 



Though this genus is already well characterized, I have made upon 

 it some remarks which may not be without interest. 



Like the other genera of the family of Pennatuloid Polyps, they 

 have a soft distinct axis, with a terminal kidney -shaped flat disk, 

 spreading horizontally upon the vertical stem, so that. its plane is at 

 right angies with the stem, though the two halves are symmetrical 

 upon the longitudinal diameter of the upper part of the axis. The 

 axis, and also the disk, are hollow, and their cavities communicate 

 freely with each other. In their expanded state, these cavities are 

 filled with water by means of which they swell to a very remarkable 

 extent, so much so that the disk may be enlarged to twice its diame- 

 ter, and the peduncle of the stem to four times its height in an expan- 

 ded state, when compared with its utmost state of contraction. 

 When expanded, the stem stands upright, sunk in sand, its lowest 

 extremity swollen into an oblong bulb, somewhat compressed late- 

 rally. The middle part of the stem is more cylindrical ; but in its 

 upper part, it again enlarges into a sort of funnel, compressed late- 

 rally, extending with its prominent diameter, under the lower sur- 

 face of the disk. The disk itself, is kidney or heart shaped, thicker 

 in the middle, when swollen, and growing thinner towards the edges. 

 Its lower surface is smooth at the stem, but there is a sort of radia- 

 tion observed upon it, extending from the summit of the stem 

 towards the margin, and corresponding to the linear radiating arrange- 



