342 
POPULAR SC ENCE REYJEW. 
with prominent groups of poison-bearing thread-cells, which give 
it a pretty beaded appearance and a deadly touch (fig. 7, t). 
When the medusa swims, the arms are coiled in a spiral, and 
are borne “ tightly twisted like a corkscrew ; r ’ when it is at rest 
they hang passively pendent, or are cast out to several times 
the length of the bell, and float in undulating lines through 
the water. The opening of the bell below is closed by a mem- 
branous film (the velum), with a circular orifice in the centre, 
through which the water finds access to the interior. This 
veil is a continuation of the muscular layer which lines the entire 
cavity of the swimming-bell, and endows it with its remarkable 
contractility. 
There only remain to be noticed the simple organs of sense 
with which the locomotive zooids of the Kydroid are generally 
furnished, but which of course are altogether wanting amongst 
the fixed and vegetative members of the colony. These are of 
two kinds : the first consists of a collection of pigment-cells, 
forming a coloured spot or ocellus, and inclosed by a deli- 
cate membrane (figs. 7, 8). In some cases a crystalline body, 
a refracting lens, is embedded in the pigment-mass, and we are 
naturally led to regard the whole structure as an eye. Even 
when the lens-like body is wanting, the elements that remain 
may possibly constitute a light-perceiving organ of the simplest 
and most rudimentary kind. The ocelli, which are often dark- 
red or black, are borne on the bulbous enlargements from which 
the tentacles spring (fig. 3). The remaining organs of sense 
that occur on the medusa are certain minute sacs developed on 
the margin of the swimming-bell, in each of which one or more 
refractile spherules are inclosed (fig. 2 ; in this species they 
are very numerous). They have been regarded as organs of 
hearing, from their supposed resemblance to the auditory sacs 
that occur in other classes ; but there seems to be no real 
analogy between the two, and accordingly some of the ablest 
observers assign them a visual function. It may be impossible 
to determine the precise significance of these primitive struc- 
tures ; but we shall hardly err in viewing them as sense-organs 
of the simplest type correlated with the habits and needs of a 
free and active existence. Though almost universally present, 
there are a few known cases in which they seem to be wanting. 
The medusa represented in our fig. 3 — a remarkable form, which I 
have lately obtained, and which is still undescribed — is destitute, 
at least in its earliest stage, of both ocelli and marginal sacs.* 
The nervous system of the medusa, if such should exist in 
any specialised form, has certainly not been detected in the 
vast majority of known Hydroids ; and though such a system 
* So far as is known at present, the two sense-organs (the ocellus and 
the marginal sac) never exist together on the same medusa. 
