THE EARTHWORM 301 



are the three so-called giant fibers, the function of which is proble- 

 matical. The entire cord is surrounded by a connective tissue 

 sheath in which are included muscle fibers and the sub-neural 

 and lateral neural blood vessels, which extend throughout the 

 length of the body. The earthworm's body, therefore, consists of 

 cells and cell products, as do the bodies of other animals. Epi- 

 thelial, muscular, sustentative, and nervous tissues, with their vari- 

 ous types of cells, are represented, although the modifications of 

 these principal types are less diversified than in the bodies of higher 

 animals. Likewise, the varied functions of the body are cell func- 

 tions. 



Metabolic Processes. — In the earthworm, the processes by 

 which the nutrients are digested, absorbed, and conveyed to the 

 cells of the body, and the excretions eliminated, resemble the corre- 

 sponding processes in the frog, since there exists in each of these 

 animals a circulatory sj'stem, in which the blood is everywhere 

 confined to definite channels. Within the tissues of the earth- 

 worm there are likewise intercellular lymph spaces resembling 

 those of vertebrate animals. While the arrangement of the blood 

 vessels and the nature of the blood differ markedly in vertebrates 

 and annulates, the fundamental relationships are alike: nutrient 

 material is absorbed from the digestive tract into the blood and 

 thence conveyed to all parts of the body, w'here the assimilation and 

 dissimilation occur within the cells, the result being that the waste 

 products of metabolism are returned to the blood, which conveys 

 these substances to the excretory organs. 



Irritability and Behavior. — Earthworms respond to various 

 forms of stimulation. If the term " sense " may be appHed to 

 animals of such a lowly organization, they have " senses " of 

 touch, taste, smell, and temperature; and, although lacking a 

 sense of hearing like our own, they are extremely sensitive to 

 mechanical vibrations in solid objects with which they may be in 

 contact. They are also sensitive to light, as shown by their 

 retreat into their burrows when a light is suddenly turned upon 

 them, and by their nocturnal habits. In the volume on " Vege- 

 table Mould and Earthworms," to which reference has already 

 been made, Darwin described their responses to various forms of 

 stimulation as observed in confinement and in nature. Despite 

 the limitations of any study of animal behavior published so many 

 years ago, the painstaking observations made by this great natural- 



