30 



LIFE, IN ITS LOWER FORMS. 



considered to be plants, in spite of their movements, and 

 constitute the order Diatomaceas. But what is stranger 

 still is, that there are some forms which are animals at 

 one period of their lives and plants at another ! The 

 Green Microglene {Microglena monadind), a beautiful oval 

 monad not uncommon in our ditches, is declared by Kiitz- 

 ing to be produced from a threadlike plant, which he 

 names Ulothrix gonata. From the cells of which the thread 

 is made up, the minute vegeto-animals are discharged in 

 numbers (See Plate II. fig. 1 a), and assume the form 

 of an oval green monad, with a red eye-speck, a transpa- 

 rent colourless mouth, and a delicate proboscis or cilium 

 (6). They swim energetically, with a vibratory rotation on 

 the long axis ; increase by self-division (c) ; and at length, 

 by transverse constriction and elongation (d), grow into 

 jointed vegetable threads (e), the lowest joint still retain- 

 ing the eye-speck. 



This interesting phenomenon, the reality of which has 

 been ascertained by Kiitzing beyond all possibility of doubt, 

 dissipates the idea of any supposed line of demarcation 

 between the organic kingdoms of nature ; and proves that 

 the disputes which have been so pertinaciously maintained 

 between zoologists and botanists on their boundary ques- 

 tion, have been concerning words rather than things. 



Among the organisms the position of which has been 

 most debated, are some very familiar to us, from our 

 habitual employment of some of the species for domestic 

 purposes. They constitute the extensive and widely- 

 distributed class Porifera, or the Sponges, the history of 

 which forms the subject of this chapter. We shall not 

 enumerate the names or record the opinions of the contro- 



