xxii Introduction. 



nised as belonging rather to the Mollusca, and the Sponges to a 

 distinct and lower group of animals than the 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 Acalephae 

 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 CCELENTERATA has been given. They are 

 divided into two classes : I. The ANTHOZOA, or Polyps, in- 

 cluding the orders Helianthoida and Asteroida ; and II. The 

 HYDIIOZOA, composed of the Hydroid Polyps and Acalephae, 

 the connection between which, as indicated in the text (p. 609), 

 is very intimate. 



V. Tlie 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 Rotifcres, 

 and the other Les Infusories homogenes,l>Mt 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 Sponge*, 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. 



