MEDUSA BIFIDA. 119 



While in this state, such spontaneous influence is exerted over the 

 whole specimens, that the contraction of the cylinder shortens the interval 

 between the circles, or the whirls, if they be prominent, compressing 

 them on the bulb. As the cylinder relaxes, the intervals are wider, and 

 the whirls become more distinct; the subject is lengthened. 



I call the formation a cylinder or roll, as in the earlier stages it ap- 

 proaches this figure ; and it very much resembles a minute rouleau of sil- 

 ver coinage. Other subjects, farther advanced, exhibit waved instead of 

 plain circles around the pillar, roll, or cylinder. — PI. XIX. fig. 5 ; XX. 

 fig. 2. Short clefts likewise indent the convexity of the waves, especially 

 towards the row of tentacula still subsisting at the summit b. 



It must be observed that this row of tentacula, consisting of twenty 

 in some subjects, is remote from a, the smooth, simple, fleshy bulb. 



The number of circles, whirls, or wavings of the cylinder are indefi- 

 nite. All are not formed at once, and by the result it has been computed 

 there were twelve, fourteen, twenty, or more ; as many as twenty-seven 

 seemed to belong to one specimen. The number augments : a plain whirl 

 next the bulb may be converted to a waving circle in a day or two, or less. 

 The advance towards change or maturity is always progressively upwards. 

 Thus the portion of the cylinder next the bulb may be deemed an elemen- 

 tary stage, and the farthest from it the most perfect subsisting stage. 



Desirous of pursuing the history of individuals from earlier date to 

 the latest period, I detached several single specimens thus affected, by in- 

 sinuating the point of a stout pen-knife, so as to split off a fragment of the 

 shell, which was little larger than the base of each subject. This could 

 be attached by some adhesive composition to a watch-glass, and suspended 

 in a convenient position. 



Several being obtained in this manner, all were isolated. They were 

 such as Plate XIX. figs. 4, 5 ; and Plate XX. fig. 2 ; the last two being 

 farther advanced than fig. 4, the first referred to. 



The whole consisted of a plain adhering bulb, sustaining a prolonged 

 roll or cylinder, of which the extremity was crowned by a circular row of 

 tentacula. 



These tentacula were not alike vigorous and active in all the subjects. 



