INFUSORIA. 19 



rulers upon a table, they may form a square or parallelogram. 

 The motion is peculiar, each single body (the edges remaining in 

 a line) being capable of sliding rapidly along the one with which 

 it is in contact ; so that when those upon one side have extended 

 themselves, the whole may resemble a flag attached to a pole, as 

 in the diagram. 



The Gaillonellm (plants) are united end to end, forming a 

 chain. In the genus Meridion the individual ])arts or frustules are 

 wedge-shaped, so that when placed edge to edge they form an arch, or even 

 a circle or spiral. In Micrasterias the interior is divided into many cells. 

 Istlmiia has an end hxed, and forms rows, the parts being in contact at 

 different angles by a small isthmus. 



All the animal Infusoria seem to be j^rovided with a mouth, which is 

 generally terminal, but sometimes placed near the middle of the body. The 

 vent is not always present, or at least has not been discovered in all, so that 

 it is possible that the undigested portion of the food may be rejected from 

 the mouth, as in the Polypi. Where it has been observed, the vent was 

 near the mouth ; or upon the abdomen ; or at the posterior extremity, 

 above or below. The breathing organs, where they have been observed, 

 appear as simple openings. The organs of motion are the tail, foot, bristles, 

 vibrillae, &c. 



The organs of sense, as far as known, are those of feeling (of which tlie 

 snout, and perhaps the bristles, are the organs), and perhaps vision, although 

 Dujardin doubts the existence of the latter. The organs supposed to be 

 eyes are dark red or black stigmas situated anteriorly upon the upper side, 

 and Ehrenberg thinks that a glandular body beneath them performs the 

 function of an optic nerve. Most of the polygastrica have a single stigma, 

 Distigma has two ; some of the Rotifera (Crustacea) two, three, four, or 

 more, arranged in two clusters, as in Theorus ', in a semicircle, as in 

 Gyclogena / or upon pedicles (like those of land snails) as in Otoglena. It 

 is uncertain whether the structure of these eye-like spots resembles that of 

 any of the animals above them ; or whether their vision, if they possess 

 this sense, is more than sufficient to distinguish light from darkness. Indeed, 

 a perfect vision would scarcely be of much use to them, as they are said 

 not to sleep, but to be as active in darkness as in light. 



The internal organization of these singular animals is not less interesting 

 than their exterior form ; and indeed it may be considered the more sur- 

 prising, when their size, in comparison with that of the larger animals, is 

 taken into consideration. Their movements are extremely multifarious, 

 and for all these there is an appropriate system of muscles. These may be 

 observed running in various directions, the most interesting being those 

 which keep up the unceasing play of the vibrillae But these organs arc 

 not confined to this class, vibrillse being found externally and internally in 

 many of the higher animals. 



In the Polygastrica Ehrenberg represents a long curved intestine with 

 numerous globular bodies suspended to it somewhat like grapes, from the 



22;j 



