104 Z. HYDROIDA. Hydra. 



But the Hydra is principally celebrated on account of its manner 

 of propagation. It is of course like zoophytes in general, asexual; 

 and every individual possesses the faculty of continuing and multiply- 

 ing its race, principally, however, by the process of subdivision. 

 During the summer season, a small tubercle rises on the surface, 

 which lengthening and enlarging every hour, in a day or two de- 

 velopes in irregular succession, or in successive pairs,* a series of 

 tentacula, and becomes in all respects, excepting size, similar to its 

 parent. It remains attached for some time, and grows and feeds, and 

 contracts and expands after the fashion of this parent, until it is at 

 length thrown off by a sort of sloughing or exfoliation. These buds 

 sprout, in the common species, from every part of the surface of the 

 body, but not from the tentacula; and very often two, three or four 

 young may be seen depending at one time from the sides of the fruit- 

 ful mother, in different stages of gi'owth, every one playing its part 

 independent of the others : 



" where some are in the bud, 



" some green, and rip'ning some, while others fall." 



They are evolved with rapidity in warm weather especially, one 

 no sooner dropping off than another begins to germinate ; " and 

 what is most extraordinary, the young ones themselves often breed 

 others, and those others sometimes push out a third or fourth genera- 

 tion before the first fall off from the original parent." — Trembley 

 found in one experiment that an individual of H. grisea produced 

 forty-five young in two months ; the average number per month in 

 summer was twenty, but as each of these began to produce four or 

 five days after its separation, the whole produce of a month was pro- 

 digious, f 



" No sooner is a young one furnished with arms, than it seizes and 

 devours worms with all possible eagerness ; nor is it an unusual thing 

 to behold the young one and the old one struggling for, and gorging 

 different ends of the same worm together. Before the arms come out, 

 and even sometime afterwards, a communication continues between 

 the bodies of the old and young, as appears beyond dispute by the 

 swelling of either when the other is fed. J But a little before the 

 young one separates, when its tail-end begins to look white, trans- 

 parent, and slender, the passage between them, I believe, is closed. 

 And when the young one comes away, there remains not the least 



Baker's Hist. 35. 

 f Mem. pour I'Hist- des Polypes, 174 — 5. Also Baker, lib. s. cit. 53 — 4. 

 ^ By some clever dissections, Trembley demonstrated the reality of this com- 

 munication. Mem. 161-2. 



