Royal Physical Society. 257 



2. Trichydra pudica. 

 Shells and stones which have been kept quiet in an aqua- 

 rium for some time, are occasionally covered with a flocculent 

 net-work of shining fibres, which appear as fine as the lines of 

 a spider's web. This net-work, under microscopic power, is 

 found to be composed of the interlacing tentacles of a multi- 

 tude of closely-congregated polyps, attached together by a 

 linear creeping polypary (Plate XV., fig. 1). The polyp of this 

 minute Zoophyte (which I have called Trichydra pudica, " the 

 modest hair polyp") is about ^th of an inch in length, and 

 resembles in shape a miniature fresh-water hydra. The whole 

 body is exceedingly attenuated and transparent, with the ex- 

 ception of the buccal cavity, which is of a dense silvery white, 

 and may be distinguished by reflected light as a shining speck, 

 while the rest of the animal is almost invisible. The tentacles 

 vary in number from 4 to 12, with the increasing age of the 

 polyp. They are arranged in a single row, and are long and 

 waving, and muricated with clusters of minute thread-cells, 

 above which project long and finely acuminated " palpocils," 

 the soft prehensile spines I have described in former com- 

 munications. The buccal cavity is small and conical, and occu- 

 pies a scarcely elevated papilla situated in the centre of the 

 tentacular circle. Its walls are exceedingly dense, and open 

 superiorly by five motile lips. The buccal cavity is frequently 

 everted as a flat disk, when the tentacles are depressed along 

 the body /. For a long time I considered that the polyps were 

 naked and single, as I was unsuccessful in detecting either a 

 connecting polypary or a corallum, while the Zoophyte re- 

 mained in situ, and any attempt to remove it caused the polyps 

 to disappear altogether. Afterwards , the stones on which they 

 grew became coated with fine dust, deposited from the water, 

 and afforded no hold for the creeping polypary ; the latter, 

 therefore, floated unattached as tortuous white threads bearing 

 polyps. The polypary was inclosed in a transparent mem- 

 branous sheath or corallum, which at intervals bore short, 

 cylindrical, even-rimmed cells of unequal length, for the re- 

 ception of the polyps. This interesting little zoophyte is re- 

 markable for the laxity of its habit, and the extensibility and 



