6 MANUAL OF ZOOLOGY. 



nucleated, is the formal basis of all life," * there, nevertheless, 

 remain certain conditions equally indispensable to the external 

 manifestation of vital phenomena; though life itself, or the 

 power of exhibiting vital phenomena, may be preserved for a 

 longer or shorter period, even though these conditions be ab- 

 sent. These extrinsic conditions of vitality are, firstly, a cer- 

 tain temperature varying from near the freezing-point to 1 20 

 or 130 ; secondly, the presence of water, which enters largely 

 into the composition of all living tissues ; thirdly, the presence 

 of oxygen in a free state this, like water, appearing to be a 

 sine qua non of life, though certain fungi are stated to offer an 

 exception to this statement.t 



The non-fulfilment of any of these conditions for any length 

 of time, as a rule, causes death, or the cessation of vitality ; but, 

 as before remarked, life may sometimes remain in a dormant 

 or " potential." condition for an apparently indefinite length of 

 time. An excellent illustration of this is afforded by the great 

 tenacity of life, even under unfavourable 'conditions, exhibited 

 by the ova of some animals and the seeds of many plants ; but 

 a more striking example is to be found in the Rotifera, or Wheel- 

 animalcules. These are minute, mostly microscopic creatures, 

 which inhabit almost all our -ponds and streams. Diminutive 

 as they are, they are nevertheless, comparatively speaking, of 

 a very high grade of organisation. They possess a mouth, 

 masticatory organs, a stomach, and alimentary canal, a dis- 

 tinct and well-developed nervous system, a differentiated 

 reproductive apparatus, and even organs of vision. Repeated 

 experiments, however, have shown the remarkable fact that, 

 with their aquatic habits and complex organisation, the Roti- 

 fers are capable of submitting to an apparently indefinite de- 

 privation of the necessary conditions of their existence, without 

 thereby losing their vitality. They may be dried and reduced 

 to dust, and may be kept in this state for a period of many 

 years ; nevertheless, the addition of a little water will at any 

 time restore them to their pristine vigour and activity. It 

 follows, therefore, that an organism may be deprived of all 

 power of manifesting any of the phenomena which constitute 



* It has not yet been shown that the living matter which we designate 

 by the convenient term of " protoplasm " has universally and in all cases 

 a constant and undeviating chemical composition ; and there is, indeed, 

 reason to believe that this is not the case. It is also certain that there are 

 other materials, the exact use of which we do not at present know, which 

 are absolutely essential to the maintenance of life, probably even in its 

 humblest manifestations. 



t Recent experiments, as yet unconfirmed, would go to prove that these 

 conditions of vitality are not of such essential importance. 



