JELLY-FISHES. 315 



resumes its energetic pumping, and shoots round and 

 round in an oblique direction, the summit always going 

 foremost, and the tentacles streaming behind in long trail- 

 ing lines. Now it is again arrested ; the bell turns over 

 on one side and remains motionless, and the tentacles, 

 " fine as silkworm's threads," float loosely in the water, 

 become mutually intertangled, instantly free themselves, 

 pucker and shrivel up, slowly lengthen, and hang motion- 

 less again, or, as the bell allows itself to sink slowly, are 

 thrown into the most elegant curves and arches. 



Though these tentacles look at first like simple threads 

 of extreme tenuity, yet when viewed closely they are seen 

 to be composed of a succession of minute knobs separated 

 by intervals, like white beads strung on a thread ; the 

 beads being more remote from each other in proportion as 

 the tentacle is lengthened. 



This structure is worthy of a more minute investigation. 

 We will therefore confine our little Sarsia in this narrow 

 glass trough, which is sufficiently deep to allow its whole 

 form to be immersed, though somewhat flattened ; which 

 is an advantage, as its movements are thereby impeded. 

 Now, with a power of 300 diameters, you see that each of 

 the knobs of the tentacle is a thickening or swelling of the 

 common gelatinous flesh, in which are imbedded a score 

 or two of tiny oval vesicles, or bladders, without any very 

 obvious arrangement ; but for the most part so placed that 

 the more pointed end of each is directed towards the cir- 

 cumference of the thickening. The intermediate slender 

 portions of the tentacle the thread on which the beads 

 are strung is quite destitute of these vesicles. 



These little bodies are called cnida ; and, in the whole of 

 this class of animals, and also in that of Zoophytes, they 

 play an important part in the economy of the creature. I 

 shall probably take occasion to exhibit them to you under 

 conditions more favourable than are presented here, viz., 

 in the Sea-Anemones, where they attain far greater dimen- 



