36 



Aquatic JLitt 



from the evidence at hand approximately 

 what these links were. 



After the formation of the earth, when 

 the crust had cooled sufficiently to allow 

 the vapors in the air to condense upon 

 it, so forming the oceans, it is supposed 

 that the first forms of life which appeared 

 were unicellular in character, that is, 

 composed of single cells — the lowest 

 form in which life can exist. It has been 

 assumed that of the two kingdoms, plant 

 and animal, the former was the first to 

 appear as plants are able to live at higher 

 temperatures and can subsist upon an 

 entirely inorganic diet; animals, on the 

 other hand, require an organic source of 

 food supply. It is reasonable then to 

 suppose that the plant kingdom was the 

 forerunner of the animal kingdom. 



When animal life first appeared the 

 simplest forms prevailed, namely, the 

 Protozoa or single-celled animals. These 

 •varied in complexity from the very sim- 

 ple Amoeba to the more organized 

 Infusoria and highest of all Protozoa, 

 the Tentaculifera. During the subsequent 

 ages the forms of life gradually increased 

 in complexity by very slow steps from 

 the single cells, of which there were al- 

 most countless numbers of species, to the 

 sponge group which is the link from the 

 highest Protozoa to forms like the hydra 

 and the sea-anemone, which are among 

 the first animals to possess a digestive 

 cavity. All animals so far were com- 

 posed of only two elementary layers of 

 tissue ; an outer layer or ectoderm and 

 an inner layer or endoderm. The next 

 highest existing group is the first to pos- 

 sess a third elementary layer of tissue, 

 this being a middle layer or mesoderm. 

 An example of this group is the planarian 

 worm, which is about one quarter to 

 three-eighths of an inch long, resembling 

 very much in outward appearance a small 

 leech and generally found around decay- 



ing vegetable matter in ponds. Up to 

 this stage all the existing animals are in- 

 vertebrates, or, in other words, do not 

 possess a backbone. A vertebrate is an 

 animal with a backbone or spinal column, 

 having a spinal cord or great nerve lying 

 above it. It is to this group that man 

 and all the higher animals belong. One 

 of the first animals which shows a ten- 

 dency toward vertebrate structure is the 

 lancelet. This is not a true vertebrate, 

 but forms one of the very few connecting 

 links existing today between vertebrates 

 and invertebrate. This fishlike animal, 

 about two inches in length, lives almost 

 completely imbedded in the sand on the 

 sea bottom. Along its back passes a 

 notocord or primitive backbone, and 

 above this lies the spinal cord. The noto- 

 cord is soft, but sufficiently strong to act 

 as a support for the body. This brings 

 us to the fish proper. 



One of the most representative of the 

 fishes is the perch, consequently a great, 

 deal of the following description relates 

 to it primarily, but may also be applied 

 to a great many other species. We will 

 first consider the external appearance. 

 The body is elongated, compressed from 

 side to side, and tapers at both ends. It 

 is divided into three parts, the head, 

 trunk and tail. One of the first things 

 that strikes our eye as we look at a fish 

 are its fins. These are generally five in 

 number ; three unpaired and two paired 

 ones. The unpaired ones are the dorsal, 

 caudal and anal fins ; the paired ones the 

 ventral or pelvic and the pectoral fins. 

 There is present in some fishes another, 

 called the adipose fin, situated on the 

 dorsal surface between the dorsal and 

 caudal fins. It is usually small. The 

 dorsal (i and 2) and anal (3 and 5) 

 fins are divided into two parts, the spin- 

 ous (1 and 5) and the soft (2 and 3) 

 portions. These two parts are supported 



