292 THE NORTHERN MICROSCOPIST. 
only descriptive of the former class which has been subdivided into seven orders, 
of which the typical one, the Hydridze, contains the single genus Hydra, all the 
members of which are free swimming single animals. ‘The other orders of the 
Hydrozoa (except the jelly fish) are social animals, and live in clusters or 
colonies, produced by a process of budding from a single individual. The 
body of the Hydra consists of a gelatinous sac, capable of varying its shape, 
having one end extended into a disc or foot; the other being occupied by the 
mouth, which leads into a capacious cavity, occupying the whole of the interior 
of the animal. The mouth is surrounded by a circle of tentacles or feelers, 
which vary in number, and are capable of altering their shape from being short 
and stumpy to long and filiform. This power is also possessed by the entire 
body of the animal, and is due to the presence between the two layers of cells, 
composing the body of a layer of muscular cells or fibres. The size of the 
fTydra varies from less than a quarter to three-quarters of an inch. The walls 
of which the body is composed are cellular in structure and of sarcode sub- 
stance. The outer layer of cells known as the ectoderm are furnished with 
nematocysts or thread cells, each of which contains a long thread which is 
hollow and connected with a poisonous secretion, contained in the cells believed 
to be formic acid. When the tentacles are touched, these threads are ejected 
and piercing, the bodies of the minute objects upon which the Hydra feeds 
cause the death of the prey and enable the Hydra more readily swallow it. 
The jelly fish, which are believed to be very largely transition states of massive 
hydroid forms are endowed with similar cells, hence the stinging sensation felt in 
handling one of them. ‘The inner cells, or endoderm, are lined with vibratile 
cilia, which coupled with the amceboid motion of the cells themselves pass the 
food along the different parts of the body cavity and so aid digestion. No 
nerves have been discovered in the Hydra, which yet appears to be endowed 
with great sensibility, shewing the extreme probability of the theory that the 
nerves and nerve centres in the higher animals are only aggregations of particles 
which in the lower animals receive and convey impression in the molecules of 
which the cells are composed. The Hydra possesses an extraordinary degree 
of recuperative power. It may be cut into segments and otherwise mutilated 
without its being injured, except to its advantage, for the mutilations are 
speedily repaired, and the Hydra appears to be more productive near the 
mutilated parts, and the segments into which any Hydra has been divided 
speedily become complete Hydra themselves. The earlier investigators of 
the Hydra, Trembley, Barker, and others, have left on record very remarkable 
accounts of their experiments, which about the middle of the eighteenth cen- 
tury excited great interest. The modes of reproduction are by ova, and by 
budding: the latter mode generally occurs in warm weather. Hydra, similar in 
all respects to their parent, seem to sprout from its side, and perhaps themselves 
to bud, so that three generations of Hydra may be seen attached together. 
There are three species of Hydra, one of which Hydra viridis is green, and the 
other two of which Aydra fusca and Hydra vulgaris are brown. Related to 
the Hydra are the Sertularize, often found attached to sea-weeds and rocks, a 
number of which were described by the lecturer. Many of the Meduse, or 
Jelly-fish, are believed to be transition stages of Hydra, and in one case, that of 
Chrysaora hysocilli, the complete life history has been traced, and is a most 
remarkable one, every different stage being of such a character as to induce an 
observer to believe it a separate animal. The Zoophytes have been objects of 
study for centuries, but until 1599 were believed to be either plants or inanimate 
objects, the advocates of the latter opinion holding that the polyps. were 
simple evidences of fluorescence or crystallization. In 1599, Ferank Imperato, 
a native of Florence, claimed the Zoophytes as animals. He was followed later 
by Pegsonnel, in 1727. The Hydra having meanwhile been discovered by 
Leeuwenhoek, The animal theory was received with ridicule until 1741. 
Trembley’s experiments, backed up by the opinion of Reaumur and others, 
