66 ZOOPHYTES. 



lower part of the head. A prominence on the neck of a hydra, Plate XI. 

 fig. 4, exhibited similar currents, though less distinct. 



The rate of such currents is not altogether uniform. This, indeed, is 

 seen in the Ascidia, and various kinds of the lower animals ; and in many, 

 their acceleration or retardation must result from the mutable form of the 

 subject. Sometimes the particles are carried along with considerable velo- 

 city in the hydra now described. 



This hydra has no power over the skeleton, which, in comparison 

 with it, must be considered rigid. 



The same vegetative faculty distinguishes the present species as many 

 Sertularise, for, having cut two portions from a specimen, I found them 

 rooted to the glass whereon they lay in the course of a night. 



Medusa ocilia. — Practical naturalists must be well aware that nume- 

 rous colonies of minute Medusae, and even successive generations of them, 

 sometimes appear in vessels containing various marine collections. But 

 it is extremely diflScult to pursue the history and progress of such trans- 

 parent, unmanageable, and too often evanescent beings. Their sudden 

 existence has surprised me again and again. I was unable to ascertain 

 their origin ; they have remained some days in activity, then vanishing 

 without leaving the slightest traces behind. No species could be pre- 

 served with facility, though for only a short period ; they did not seem 

 referable to any particular time or subject. Every thing regarding them 

 was wrapped in mystery. 



The smaller the objects, indeed, and the less conspicuous their aspect, 

 independent of the delicacy of their perishable nature, the more restricted 

 the scope for permanent observation. Yet there are some so hardy, and 

 so readily beheld, not a tenth part of the dimensions of their fellow-tenants 

 of the waters, which may remain so long as to disclose their own his- 

 tory. 



While a number of vigorous hydrse still terminated the extremities 

 of the zoophyte above described as the Tubidaria ramosa, I suspended 

 various specimens by silk threads, in vessels of sea-water. This is a method 

 to be particularly recommended for convenience and security ; the subject 



