xxii Introduction, 
nised as belonging rather to the Mollusca, and the Sponges to a 
distinct and lower group of animals than tlie Kadiata; the re- 
mainder have generally been divided into the following three 
orders : — 
1. Helianthoida. This order includes the actinia, or sea-anemone ; and 
the madrepores, sea-mushrooms, and brainstones, which live in 
communities, and possess the power of secreting calcareous 
matters, which they emit to form these stony substances. 
2. Asteroida. Some of the animals belonging to this division are 
called sea-pens, and others form some of the different kinds of 
coral, particularly that used for necklaces, &c. 
3. Hydroida. This order includes the fresh-water polypi, which, it is 
well known, by the experiments that have been tried, may be 
cut in pieces and even turned inside out without destroying 
life. It must be observed that the contents of this group in 
Cuvier's system consisted of all those forms of animals which he 
could not, in accordance with the knowledge possessed in his 
day, conveniently place anywhere else. Within the last few 
years, however, great progress has been made in the arrange- 
ment of the animals placed in this group by Cuvier. One of the 
most important changes has been the establishment of a fifth 
group of animals for the Infusoria and Sponges, together with 
certain other creatures of very low organisation. To these the 
name of Protozoa has been given. The Entozoa have been 
removed amongst the articulate animals, and there is a growing 
conviction that the Echinodermata will have to be transferred 
to the same section. There remain, consequently, the Acalephce 
and Polyps of Cuvier, which form a group characterised by 
their soft and generally gelatinous texture ; by the existence of 
peculiar cells, called thread cells, in the skin ; and by their pos- 
session of an alimentary cavity with only a single orifice. To 
these the name of Ctclenterata has been given. They are 
divided into two classes : I. The Anthozoa, or Polyps, in- 
cluding the orders Helianthoida and Asteroida ; and II. The 
Hxduozoa, composed of the Hydroid Polyps and Acalephae, 
the connection between which, as indicated in the text (p. 609), 
is very intimate. 
V. The Infusoria, or Animalcula, are so small as to be invisible to the 
naked eye, and they are all inhabitants of liquids. Cuvier ar- 
ranged them in two orders, one of which he called Les Rotiferes, 
and the other Les Infusories homogenes, but the first of these divi- 
sions is now included among the Articulata. The remainder of 
the Infusoria of Cuvier, with the exception of some which are 
now known to be of vegetable nature, are arranged, with the 
Sponges and some other animals, in a separate division, called 
Protozoa, the classification of which is still in a somewhat un- 
certain state. The three principal classes are those of the Infu- 
soria, the Sponges, and the Rhizopoda ; but there are other forms 
which will not admit of being brought under any of these deno- 
minations. Nearly all the Protozoa are microscopic, except 
when, as in the case of the Sponges, they form an aggregation 
of individuals. They are very numerous, and, although ex- 
ceedingly simple in their structure, their history often possesses 
much interest. 
