122 DESCRii'TxoN Of \^Poli/(jastrica. 



invisible wlien empty, but when distended mth a limpid fluid, 

 appear like so many lucid resides within the creatures. 



The propagative ajjparatus has been particularly noticed in the 

 species M. guttuJa and M. viviimra. It consists of a vast number of 

 granules formed into a net-like mass, and dispersed generally 

 throughout the creature, and of a comparatively large spherical 

 glandular body, which separates by the process of self-division. 



Monads propagate also by another method, namely, by a self- 

 diyision of the creature, either transversely, as with the Monas 

 gidtula, M. hi/aUna, M. gliscens, M. Okenii, and 31. soeialis ; or longi- 

 tudinally, as with the II. punctum (see fig. 2) ; both methods have 

 been observed in M. tivipara. The formation of gemmules has not 

 been perceived in this genus. 



Ehrenberg supposes that they are endowed with the faculty of 

 sensation, and that this is shewn by the alternate vibration and 

 quiescence of the proboscis, when the creatm-e is in a place abun- 

 dantly supplied with food. In some of the species an eye-like organ 

 has been discerned, but as the species of each genus shoiild be 

 reduced to the rule of a special organ, characterizing a particular 

 genus, these are not considered as true Monads, but form distinct 

 genera, as Microglena, &c. 



As the Infusoria of this genus are chiefly curious on account of 

 their extreme minuteness, and in no other respect, the species are 

 not deemed of sufficient interest to be expatiated upon at any great 

 length ; only their leading chai'acters and size, are therefore given. 

 And we may I'emark, generally, that most of them ai-e inhabitants 

 of water in which organic matter is undergoing decomposition. 



The Monads are arranged imder tAVo divisions, according to their 

 external forms. The fiLrst division contains all those of a globular 

 or oval shape (globular Monads) ; second those of a lengthened fonn, 

 more than twice as long as broad (elongated Monads). 



A. — Globulae Monads. 



{a). True Glohilar or Sphero- Monads — colourless or whitish. 



MoKAS crepusculum — The twilight Monad. — The animalcules of this 

 species are the smallest of all living creatm-es. They are of a spheroidal 

 fonn and hyaline, although, when seen in masses, with the naked eye, 



