16 ' INTRODUCTION. 



epidermal appendages. The serpent casts its skin, the 

 bird its feathers, crabs and lobsters their claws, just as the 

 leaves and bark fall from the branches and stems of trees. 

 Moreover, the exercise of the reproductive function which 

 in man is not limited to any particular tirde, is periodical 

 in inferior animals, precisely as plants flower and fruit at 

 certain seasons of the year. At length, in the lowest 

 orders of the animal creation, the animal and plant ap- 

 proach each other so closely that it is hardly possible to 

 draw any line of demarcation between them. 



This is the case, for example, with that order of animals 

 which have been very properly called by naturalists, zo- 

 ophytes, of which the coral and the sponge are familiar 

 examples. These creatures, which show undoubted signs 

 of animality, present also at the same time many striking 



Fig. 1. 



Fig. 1. Hydra, or polype attached to a piece of stick, with its arms extended 

 in quest of prey. a. The month of the animal, surrounded hy the tentacula. 

 T>. The tendril-like grasp of an aquatic insect, c. Foot or hase of the. animal 

 with its suctorial disc. The figure shows also the natural size of the animal. 



