CLASS III. RHIZOPODA: ORDER 2. MONOSOMATA. 651 



This curious animal presents us with the essential characters of the class Rhizopoda in their 

 simplest form. 



These are all aquatic animals. Some live in fresh water, but by far the greater number in- 

 habit the sea. Although a few of them, like the Amoeba, are solitary, they consist principally 

 of associated animals ; that is to say, of masses of individu;ils, forming, as it were, a common 

 body, but each still retaining its independent existence. The class contains the two orders 

 Poli/thalamia and Monosomata. 



ORDER 1. POLYTHALAMIA. 



This term is from the Greek polus, many, and thalamos, a bed; the animals constituting. the 

 order are all inclosed in calcareous shells. These creatures are social, the shells consisting of 

 a series of distinct chambers, which sometimes communicate one with another, and sometimes 

 appear to be completely closed up; each of them is supposed to contain a separate and inde- 

 pendent animal. It is not improbable, however, that the individual animals may be so connected 

 with each other, through the medium of the openings communicating between the cells, as to 

 constitute a common mass, with which each one is partially amalgamated. 



All the Pohjthalamia inhabit the sea, and frequently occur in such numbers that the tine cal- 

 careous sand which constitutes the sea-shore, in many places consists almost entirely of their 

 microscopic coats. At former periods of the earth's history they existed in even greater profu- 

 sion than at present; and their fragile shells form the principal constituents of several very 

 important geological formations. Thus the chalk appears to consist almost entirely of the shells 

 of these animals, either in a perfect state, or worn and broken by the action of the waves ; and 

 they occur in great quantities in the marly and sandy strata of the tertiary epoch. The stone 

 which is universally employed in Paris as a building material, is almost entirely composed of the 

 fossil shells of an animal belonging to this order, the Miliola ; so that this great city, of which its 

 inhabitants used to say, that he who had not seen Paris had seen nothing, owes its architectural 

 beauties to these minute creatures, of which many thousands would not weigh an ounce. 



ORDER 2. MONOSOMATA. 



This term, from the Greek moms, one, and soma, body, includes those Rhizopoda which con- 

 sist only of a single animal. To this order belongs the Amoeba, already described, and wliich 

 may be taken as a type of the family Proteidm, which are without a covering. In the family of 

 Arcellidce, the species are inclosed in a case, composed of a mosaic of horny shell-work, this, in 

 some species, being bell-shaped, and in others bearing the form of a flask. 



THE END. 



