TUBULARIDvE. 209 



remarkable activity and seemingly spontaneous motions ; for the hypo- 

 thesis of their individual vitality is too startling to be adopted without 

 good evidence." Dr. Johnston cautions his readers against confounding 

 this sort of circulation " with those aqueous currents which flow over 

 the surfaces of the external organs of the ascidian polyps;" he adds, 

 that innumerable cilia " clothe the surfaces of their tentacula, and 

 by their rapid vibrations drive a constant equable stream of water 

 along one side, which returns along the other in an opposite direction ; 

 and by this means the purposes of respiration are effected, and the nu- 

 trient fluid fitted for assimilation with the body." 



Respecting the singular property of the head dropping off, Sir J. G. 

 Dalyell says of the Tubularia indivisa, " the head is deciduous, falling in 

 general soon after recovery from the sea. It is regenerated at intervals 

 of from ten days to several weeks, but with the number of external 

 organs successively diminishing, though the stem is always elongated. 

 It seems to rise within this tubular stem from below, and to be de- 

 pendent on the presence of the internal tenacious matter with which 

 the tube is occupied. A head springs from the remaining stem, cut 

 off very near the root j and a redundance of heads may be obtained 

 from artificial sections, apparently beyond the ordinary provisions of 

 nature. Thus twenty-two heads were produced through the course of 

 150 days from three sections of a single stem." 



One mode of propagation exhibited by these animals is that of the 

 production of what have been called by Professor Van Beneden, " free 

 or motive buds." They are produced in little clusters of bulbs, which 

 grow from the bases of the tentacles at certain seasons, and for a certain 

 period, after exclusion, possess a considerable power of locomotion. 

 Sir J. G. Dalyell informs us, that on quitting the parent the bud of this 

 species develops some little tubercles, the rudiments of the tentacles 

 ffom its under-surface, and on these, as on so many feet, moves about 

 the bottom of the water. After a time it appears to select a position 

 in which to fix its permanent abode, when " it reverses itself to the 

 natural position, with the tentacles upwards, and is then rooted per- 

 manently by a prominence, which is the incipient stalk, originating from 

 the under part of the head. Gradual elongation of the stalk afterwards 

 continues to raise the head, and the formation of the zoophyte is per- 

 fected." Other ovules undergo a certain degree of development whilst 

 still enclosed in the ovisac, and are excluded from this shelter in a form 

 somewhat resembling that of the common hydra. They then fix 

 themselves, and become gradually developed into the form of the parent 

 animal. Many polyps, apparently belonging to this family, give origin, 



