Xll 
INTRODUCTION. 
1. Tegument ary System. The skin is sufficiently distinct. 
After a few hours’ maceration in fresh water {Sag. hellis), 
the epithelial and pigmental cells are easily removed wnth 
a hair-pencil, leaving the outer layer of muscular fibre bare. 
If the specimen be immersed in spirit for a day or two 
' {A. diantims), the integument may be separated in fiakes, 
which, under the microscope, are seen to be composed of a 
multitude of short corrugated fibrill£e, set in no definite 
direction, interspersed with clear granules, pigment grains, 
and cnidte. 
An examination of the living animal {diantims, hellis, 
crassicornis, Hale, chrysanthelliim, Cor. viridis, &c.) shows 
that the skin is composed of three elements, though these 
cannot always be separated. A layer of epithelial ciliated 
cells forms the first tunic : these are constantly in process 
of being thrown off fr*om the true skin, in the form of 
mucus ; but in some cases {Pkellia, Edwardsia) they 
entangle foreign matters, and retain their cohesion as an 
investment more or less dense, and more or less firmly 
adherent to the skin. Below this is the true skin, of a 
more granular character, and carrying, imbedded in its 
thickness, a multitude of cnidae, whose discharging points 
are directed outwards. Intimately connected with this 
layer, but still lying sufficiently beneath it to be regarded 
as a distinct stratum, are tlie pigment-cells, which impart 
the colours to the animal. 
The tentacles of Aiptasia and Anthea (less conspicuously 
also of S. hellis) are lined with a dense layer of cells, forming 
to the naked eye a dark brown lining. Some peculiarities 
of these cells I have detailed (at page 167, infra) : it is 
probable that this layer may have some special function yet 
unrecognised. 
2. Mtiscidar System. In most species the muscular frame- 
work of the body is beautifully distinct, and the tissue is 
readily isolable. The column is a cylinder of muscular 
tissue, consisting of two layers, the outer composed of 
transverse, the inner of longitudinal, fibres. The trans- 
verse fibres are the more strongly marked : they average 
about ‘0001 inch in diameter, and are never striate. 
The cylinder whicli forms the column, is closed in most 
species by two extremities, which are flat, like the top and 
bottom of a tin canister : the former is the disk, the latter 
the base. Each of these is but a continuation of the same 
