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 arrangement 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 _Acalephæ_ 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 possession of an alimentary cavity with only a single orifice. To these the name of CŒLENTERATA has been given. They are divided into two classes: I. The ANTHOZOA, or Polyps, including the orders _Helianthoida_ and _Asteroida_; and II. The HYDROZOA, composed of the Hydroid Polyps and Acalephæ, 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 arranged them in two orders, one of which he called _Les Rotifères_, and the other _Les Infusories homogènes_, but the first of these divisions 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 uncertain state. The three principal classes are those of the _Infusoria_, the _Sponges_, and the _Rhizopoda_; but there are other forms which will not admit of being brought under any of these denominations. 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 exceedingly simple in their structure, their history often possesses much interest. EXPLANATION OF TERMS USED IN NATURAL HISTORY.