INTRODUCTION. 9 



jointed body and limbs, tlie external part being the hardest, and partaking 

 of the nature of an external skeleton. Thej are generally provided with 

 well developed limbs, and are able to fly, run, and swim, with great facility. 

 A few are without limbs. This division includes insects ; Crustacea, or 

 crabs ; Arachnida, or spiders ; and Annelida, or ringed worms. 



Division I. Radiata. 



Class 1. Infusoi'ia. 



This class includes animals so minute that many of them cannot be seen 

 with the naked eye. They are named from being found in infusions of 

 organic matter. They are generally provided with mhrilloi^ or vibratile 

 organs, also named cilia^ which resemble a minute fringe of hairs, which 

 are constantly in motion, causing either locomotion of the animalcula, or 

 currents of water, which bring their food within their reach. 



Order 1. Polygastrica {Homogenea^ Cuvier), to which has been attri- 

 buted a compound stomach, resembling a bunch of grapes, whence the 

 name applied by Ehrenberg, who made the supposed discovery. The 

 genus 2fonas {^fl. 74, fig. 1) is an organized globule which moves by 

 rotation. Some authors think they are not animals, but the seeds of Algse. 

 The smaller kinds are only from one to two thousandths of a line in length, 

 so that a drop of water may contain half as many individuals as there are 

 human beings upon the earth, or five hundred millions of them. 



Order 2. Rotifera, in which the action of the vibrillaj is supposed to 

 resemble a wheel in motion. They are now removed to the lowest position 

 among the Crustacea. PI. Y'i, fig. 2, Monocerca. 



Class 2. Zoophyta. 



This includes the abundant order of the coral animals, the naked Polypi 

 {fig. 4), of which the freshwater species are the best known. The sponges 

 {2)1. I'^.ffig. 3) have also been placed here. The classification given here 

 (that of^;»Z. 74) is not followed in the subsequent pages. 

 Order 1. Corallina. PI. 1^.,figs. 58, &c. 

 " 2. Gelatinosa. pi. 74, fig. 4, Hydra. 

 " 3. Actinia. PI. 11, figs. 5, G. 



Class 3. AcalepTiOB. 



This class includes the Medusse, Sea Jellies, Sea Nettles, &c. PI. 76, 

 fi^. 74, Aurelia. 



213 



