WHAT IS A PLANT ? 81 



blood-vessels to be sent to the various organs. We have seen 

 that Cuvier s argument for the existence of this cavity has failed ; 

 his deduction, we shall now see, fails also, z>., as a diagnostic 

 character, if we apply to it our strict and universal test. 



The Amceba and many of its allies, little masses of soft, 

 gelatinous matter found in our fresh-water ponds, possess neither 

 mouth nor digestive sac, but are able to take in solid food, and, 

 so to speak, form temporary " stomachs " at any part of their 

 bodies. Again, many animals are parasites, />., they reside in 

 or on other animals, feeding on their substance ; examples are 

 seen in the Gregarinidce, which are Protozoa parasitic in the 

 Earth-worm, Lobster, Frog, Rabbit, and in the intestine of the 

 much-despised Cockroach ; in the Opalina^ a Ciliate Infusiorian, 

 living in the intestine of the Frog and Toad ; in the Taenia^ or 

 Tapeworm, resident in warm-blooded Vertebrata. All these are 

 devoid of mouth and stomach, living wholly by imbibition of the 

 juices of their hosts, through their body-walls ; their food being 

 already cooked and digested for them, they need no digestive 

 cavity ; it therefore disappears, and the parasite assumes a de- 

 graded form, subsisting entirely by a " sponging " process, which, 

 together with the degradation in nature, is very often seen in the 

 " parasite " among the human race. Among the Rotifera, the 

 males are nearly all destitute of the digestive function, their 

 existence being almost wholly given to the reproductive function. 



On the other side, we now know of very many plants that 

 " digest " the solid matter of animal food, only their digestion 

 goes on outside the organism, the nourishing material being then 

 in some way absorbed. Darwin's researches on this subject are 

 well-known to all lovers of science, and are clearly set forth in his 

 wonderful book on " Insectivorous Plants." Drosera, the Sundew 

 of our peat-bogs and heaths, is an excellent example. Its red- 

 tinted leaves are covered with hair-like bodies, called tentacles 

 (they are not true hairs), whose tips secrete a viscid fluid on being 

 touched by an unwary insect alighting on the leaf The leaf-edges 

 and tentacles then bend over towards the centre of the blade, the 

 insect being now in a kind of hollow basin, held fast and sure 

 there, and bathed in the copiously-secreted fluid. After some 

 hours the leaf opens again. What has happened ? All the 



VOL. IV. G 



