104 
Z. HYDROID A. 
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 growth, 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 1’ Hist des Polypes, 174 — 5. Also Baker, lib. s. cit. 53 — 4. 
f By some clever dissections, Trembley demonstrated the reality of this com- 
munication. Mem. 161-2. 
