THE ROOT- THREAD. 141 



equal to the depth of the cell measured to the 

 rounded bottom. The tentacles themselves were 

 nearly as much more. The average length of a well- 

 grown cell is about-^th of an inch, measured from its 

 origin to its dorsal margin ; of this the cavity, and 

 the foot or tube, make nearly an equal division. 



Another specimen shows me more distinctly the 

 manner of growth. Along the deJicate frond of a 

 Rhodymenia, runs a shelly pellucid thread of excessive 

 tenacity, from which, at intervals of about a line, 

 spring up the rows of single cells. The whole appear- 

 ance reminds me of a Laomedea. The foot of the 

 first cell, at its emergence from the root thread, is 

 constricted at short intervals, so as to resemble joints, 

 or nodes. In another case the thread wanders over 

 the rock, or rather over the thin stratum of incipient 

 Coralline, which covers it. (Fig. 2.) 



No ray of phosphoric light was elicited on plunging 

 specimens into fresh water in the dark, though the 

 experiment was repeatedly tried. 



Fig. 1 represents the zoophyte of the natural size. 

 Fig. 2, the same enlarged. Fig. 3, A single polype, 

 viewed sidewise, retracted, much magnified. Fig. 4, 

 the same extruded. Fig. 5, the same, retracted, viewed 

 in front. 



THE SNAKE-HEAD CORALLINE. 



The crevices between the slanting ridges of the 

 slaty rocks at Hele fonn little angular pools, densely 

 fringed with various species of red sea-weeds, many 

 of which are of exceeding delicacy and beauty, and 



