26 ZOOPHYTES. 



the general principles. The subject of reproduction will be discussed 

 more at length under the Actinoidea. 



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 slit open soon reunites, 

 even if previously laid out flat like 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. 4lo., Leyderi, 1744 ; and Phil. Trans., 

 vol. viii. of the Abridgment, 1742. — See also Baker's Natural History of the Polype, 

 8vo. London, 1743. 



