20 



COLLEGE ZOOLOGY 



is a breaking down of protoplasm and there- 

 fore a catabolic process. 



This gaseous metabolism of the proto- 

 plasm, including absorption of oxygen, and 

 elimination of carbon dioxide, is known as 

 cellular respiration. In small aquatic ani- 

 mals, oxygen is obtained from the surround- 

 ing water, and carbon dioxide is given off 

 into the same water. In many larger animals, 

 a respiratory system is necessary to take in 

 oxygen and to expel carbon dioxide. The 

 transportation of both these gases is one of 

 the functions of the circulatory system. 



Carbon dioxide is a waste product of me- 

 tabolism, an excretion. Other waste prod- 

 ucts due to catabolic processes are water, 

 inorganic salts, and nitrogenous salts such as 

 urea. These may be cast out directly into 

 the surrounding water by small aquatic ani- 

 mals, or they may be carried by a circulatory 

 system to an excretory system, the function 

 of which is to extract waste products and 

 expel them from the body. 



Some of the energy liberated by oxidation 

 may be used in the production of substances 

 known as secretions that are of use to the 

 animal. Certain types of protoplasm may be 

 specialized for this purpose and concen- 

 trated in glands. Glands secrete sweat, diges- 

 tive juices, milk, poison, the shells of eggs, 

 and many other substances with which we 

 are familiar. They also secrete, into the 

 blood, substances that have a remarkable in- 

 fluence on our growth and behavior; these 

 are called hormones and will be considered 

 later. Biological processes involve not only 

 continual energy transformations but var}'- 

 ing energy levels. 



Irritability or excitability 



One of the fundamental properties of 

 protoplasm is its irritability. This property 

 is responsible for the reactions of an animal 

 to changes in surrounding conditions. The 

 change that brings about the reaction is 

 known as a stimulus, and the reaction as a 

 response. Most stimuli are external changes 



in the environment, but certain stimuli 

 such as hunger seem to arise from within. 

 Some of the common types of stimuli arc 

 mechanical (for example, contact), chem- 

 ical, thermal (changes in temperature), and 

 photic (for example, changes in intensity 

 or color of light). The stimulus may be and 

 often is extremely small as compared with 

 the magnitude of the response. The response 

 may depend on the nature of the protoplasm 

 stimulated; for example, it may appear as a 

 movement if muscle is excited, or as a secre- 

 tion if gland cells receive the stimulus. The 

 transmission of the excitation from one part 

 of the protoplasm to another is called con- 

 duction. Conduction is a general attribute 

 of protoplasm, but the protoplasm of nerves 

 is speciahzed for this purpose. 



CELLULAR ORGANIZATION 



Division of the 

 protoplasm into cells 



In one phylum of animals, the Protozoa, 

 the protoplasm is continuous, but in all 

 other animals the body is divided into units 

 called cells, which contain the protoplasm. 

 We owe the term cell to an Englishman 

 named Robert Hooke, who, in 1665, de- 

 scribed as "little boxes or cells" those spaces 

 surrounded by walls which he observed in 

 cork and pith with his new microscope. 

 Since the essential substance in cells is the 

 protoplasm and not the wall, the term was 

 an unfortunate choice. The protoplasm of 

 cells is of two principal kinds: (1) cyto- 

 plasm and (2) nucleus. A cell may be de- 

 fined as a small mass of protoplasm consist- 

 ing of cytoplasm and a nucleus, which are 

 enclosed by membranes. 



Size, shape, and 

 number of cells 



Cells vary in size; some are extremely 

 small, for example, blood parasites are as 

 small as ^5,000 of an inch, whereas others. 



