30 
LIFE, IN ITS LOWER FORMS. 
considered to be plants, in spite of their movements, and 
constitute the order Diatomaceos , But what is stranger 
still is, that there are some forms which arc animals at 
one period of their lives and plants at another! The 
Green Microgleuc ( Micrnglena monadina), a beautiful oval 
monad not uncommon in our ditches, is declared by Kiitz- 
ing to be produced from a threadlike plant, which he 
names Ulothrkc gonata. From the cells of which the thread 
is made up, the minute vegoto-animals are discharged in 
numbers (See Plate II. fig. 1 «), and assume the form 
of an oval green mouad, with a red eye-speck, a transpa- 
rent colourless mouth, and a delicate proboscis or cilium 
( b ). They swim energetically, with a vibratory rotation on 
the long axis ; increase by self-division (c) ; and at length, 
by transverse constriction and elongation (rl), 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- 
