344 
rOrULAIt SCIENCE EEYIEW. 
below it ; now anchored to the bottom of the vessel and casting 
out its lines in search of food. When engaged in fishing, it 
attached itself by means of a few of its tentacles, while the 
rest were thrown out in all directions and to amazing distances, 
and appeared as the finest and most attenuated threads. The 
extremities at such times exhibited a constant tremulous move- 
ment, as if they might be feeling about for the minute or- 
ganisms on which the medusa feeds. Then it would suddenly 
gather up its fishing-lines, and rise by a series of rapid jerks 
to the surface of the water. This exquisite species, as I have 
stated, has not yet been referred to a fixed stock ; it is one of 
a large company, which remains to stimulate and reward the 
researches of the zoologist. These unattached medusae are 
single chapters of as many charming biographies still unread ; 
they suggest to us the existence of a host of graceful forms, 
which, when discovered, will fill many a gap in our systems, 
and add many an interesting page to Hydroid history. 
The medusa, at the time of liberation, is in most cases far 
from having attained its perfect form. Remarkable changes 
subsequently take place in a large proportion of the kinds, 
which so completely alter its aspect that it might pass, and 
has often passed, in its adult state for a different species from 
its young self. Indeed several species have been formed out 
of the various stages of one and the same hydroid. The 
changes affect the size and form of the swimming-bell, and 
the number of many of the other organs. In fig. 5 we have 
the early, and in fig. 6 the adult state, of the same medusa. 
The deep bell of the one gives place to a flattened and ex- 
panded disc in the other. The two tentacles present at birth 
multiply into about forty, and there is a corresponding increase 
in the number of the marginal organs of sense. In some cases 
(Plate LXXXVIII., fig. 1), the change is carried still further; the 
radiating canals, as well as the tentacles, multiply, so that the 
four with which the young is (probably) furnished, are repre- 
sented by sixty or eighty in the adult. 
One of the largest of the hydroid medusae ( Zygodactlya ), 
closely related to the form represented in fig. 1, which mea- 
sures some fifteen inches in diameter and is furnished with 100 
radiating vessels, is not larger than the head of a pin in its 
early condition, has only four canals, and is moreover developed 
from one of the smallest of polypites. Growth must proceed 
rapidly in these ephemeral beings, which run their whole course 
in a single season ; they are like the annuals of the vegetable 
world, which pass through the entire cycle of their existence 
and perish in a summer. It must be noted that these changes 
are merely illustrations of the vegetative tendency to a repeti- 
tion of parts, which is characteristic of the zoophyte, and is so 
