84 



COLLEGE ZOOLOGY 



Coelom 



The coelom (Fig. 92, p. 174) is a body 

 cavity that is present in most triploblastic 

 animals; it is, by definition, a cavity or a 

 series of cavities completely bounded by 

 mesoderm. The importance of the coelom, 

 both morphologically and physiologically, 

 will be discussed later. 



Organogeny 



Organogeny is concerned largely with how 

 tissues, as structural units, are arranged to 

 make organ systems during embrj'onic de- 

 velopment. Yet it also deals with the forma- 

 tion of the specialized tissues which make 

 up organs. The characteristic tissue making 

 up an organ system, for example, the nerv- 

 ous system, is derived from ectoderm, but 

 other tissue types from other germ layers 

 are involved in the development of the 

 nervous system as a whole. 



Larvae and their 

 metamorphosis 



Many of the animals with which we are 

 familiar, such as mammals and birds, are 

 very much like their parents when they are 

 born or hatch from the egg; but among 

 lower vertebrates, such as the frogs and 

 toads, and in most of the invertebrates, the 

 animal that is born or hatches from the egg 

 is very different from its parents and is 

 known as a larva. Common larvae are the 

 tadpoles of frogs, the grubs of beetles, the 

 maggots of flies, and the caterpillars of but- 

 terflies. Many larvae do not develop grad- 

 ually into adults, but change rather abruptly 

 from the larval to the adult stage, a process 

 known as metamorphosis. Numerous exam- 

 ples of larvae and their metamorphosis will 

 be encountered in our studies of the Meta- 

 zoa. 



DIFFERENTIATION OF 

 SOMATIC CELLS: TISSUES 



Several types of somatic (body) cells can 

 be distinguished in metazoans by differences 



in shape, structure, and function; cells of 

 the same type are grouped together as a tis- 

 sue. A tissue is a group of similar cells so 

 specialized that they perform a common 

 function. The study of tissues is called his- 

 tology. Some of the simple metazoans pos- 

 sess only two kinds of somatic tissues; others 

 are made up of a great number. The many 

 different kinds of somatic tissues may be 

 classified according to their structure and 

 functions into 5 groups. 



Epithelial tissue 



Epithelial tissue (Fig. 43) consists of 

 cells which cover the surfaces of the body, 

 both without and within, such as the skin 

 and the lining of the digestive tube. It may 

 be protective, absorptive, secretive, or ex- 

 cretive in function. Epithelial tissue may be 

 flat (squamous), cuboidal, or columnar, 

 and may form a single layer or several layers 

 (stratified). It may be ciliated or noncili- 

 ated. Nutritive material may pass through 

 an epithelial tissue into the body, while ex- 

 cretory products may pass through it on 

 their way out; it may contain the end organs 

 of the sensory apparatus, and may protect 

 delicate tissues from a harmful environment. 

 Examples: epidermis and gastrodermis of 

 the hydra (p. 107), lining of coelom in the 

 frog and other animals (Fig. 213, p. 331), 

 and lining of intestine (Fig. 43). 



Connective tissues 



These tissues (Fig. 43) may be encoun- 

 tered in almost any part of the body; they 

 are the supporting or uniting structures of 

 the body. Their chief functions are (1) to 

 bind together various parts of the body and 

 (2) to form rigid structures capable of re- 

 sisting shocks and pressures of various kinds. 

 These tissues consist largely of intercellular 

 substances such as fibers, cartilage, and bone 

 produced by the cells, either within or out- 

 side the cell. The fibrous connective tissues 

 occur throughout the entire body, connect- 

 ing the cells to one another and binding the 



