26 ZOOPHYTES. 
the general principles. The subject of reproduction will be discussed 
more at length under the Actinordea. 
17. Reproduction by artificial sections may require a few words in 
this place, as it is one of the most remarkable characters of polyps, 
and is strikingly exhibited in the Hydra, as was long since shown by 
Trembley in a series of investigations pursued with wonderful skill 
and perseverance.* ‘They were cut into halves, and soon each was a 
perfect Hydra; one was divided into three parts, and in three or four 
days in summer, the tail had produced a head, the head a tail, and 
the middle part a head at one end and a tail at the other: and even 
before completion they sometimes gave out buds. From forty parts 
as many Hydras were soon formed. ‘The body shit open soon reunites, 
even if previously laid out flat hke a membrane; and new tentacles in 
a short time replace those that may be cut off. Two polyps may be 
made to change heads, for one may be engrafted on the body of 
another; and if the tail of a polyp is put into the mouth of another, 
they unite—heads and tails. It might be somewhat puzzling to 
decide the question of personal identity among such animals. Every 
portion of the animal,—unless we except the tentacles, which failed 
to reproduce a polyp in the hands of Trembley and Baker,—is capa- 
ble of forming a perfect Hydra. And this is a consequence of the 
fact that there is no general nervous centre, but each part contains a 
complete system in itself. No distinct nerves have hitherto been dis- 
tinguished. 
18. Connected with the process of growth and reproduction, there 
is a corresponding process of dying often going on in the older 
parts of a zoophyte: the polyps disappear, and the lower branches 
often drop off, leaving the trunk in this part bare. ‘These zoophytes 
are thus dying and budding in different parts at the same time. In 
the large species, the main stem or midrib of the zoophyte becomes 
lifeless, or a mere support for the numerous lateral plumes or 
branchlets. 
Besides this mode of limiting the existence of these polyps, some 
Hydroidea are said to be absorbed in their cells, and after a while to 
reappear again; and this has been observed to take place at nearly 
regular intervals. All the polyp cells of a living group have been 
found, after a certain period, empty, or with only the remains of the 
* A. Trembley, on Freshwater Polyps, 1 vol. 4to., Leyden, 1744 ; and Phil. Trans., 
vol. viii. of the Abridgment, 1742.—See also Baker’s Natural History of the Polype, 
8vo. London, 1743, 
