METAZOAX ORGANIZATION 61 



li. The Reproductive System is a closely related arrangement of 

 glands, ducts, and accessory structures which function to produce 

 new individuals of the species. Additional features of sexual repro- 

 duction are given in a separate section on that topic below. 



i. The Endocrine System includes a number of different glands 

 located in various parts of the body. These glands discharge chem- 

 ical substances, known as hormones, directly into the blood. The 

 hormones cooperate to' regulate the metabolic activity of the entire 

 body. The thyroid gland of the neck region, adrenals located near 

 the kidneys, and the islands of Langerhans of the pancreas are 

 typical examples of these organs. They go under the names of 

 ductless glands and organs of internal secretion also. 



j. The Nervous System is an organization of the nerve cell bodies 

 and their processes in such a way as to receive stimuli, carry sensa- 

 tions, correlate them, and coordinate the activities of the parts of the 

 body. By the function of the sensory portion of the system, the ani- 

 mal becomes aware of the environment and relates itself to it. In 

 vertebrates the principal parts of the system include the brain, spinal 

 cord, peripheral nerves, autonomic nerves, sense organs, and ganglia. 



The body might be thought of as being constructed by relating cells 

 to cells to form tissues, tissues to tissues to form organs, organs to 

 organs to form systems, and systems to systems to form the metazoan 

 organism. These will all be studied in more detail in connection with 

 the study of specific animals. 



Development of Sexual Reproduction 



Reproduction makes great advances among the metazoans. The 

 simple fundamental process of reproduction by cell division or 

 binary fission has been studied already. This is not possible for 

 most metazoan animals, but, in general, this type of animal begins 

 life as a single cell resulting from the fusion of two sex cells, one 

 produced by each parent. In son;e of the colonial Protozoa and 

 also in Sporozoa, as well as possibly in Paramecium, there seems to 

 be the beginning of sexual reproduction. The individuals in a 

 colony, by peculiarities in cell division, become differentiated into 

 two types: (a) the ordinary, nutritive individuals, whose means of 

 reproduction is fission and (b) reproductive individuals or gametes 

 of two forms: the large, egglike, inactive macrogametes and the 

 smaller, motile microgametes. In reproduction these two types of 

 cells unite to form a single zygote, from which a new colony arises by 



