INTRODUCTION. xiii 



of jelly fishes. Fibrous and elastic tissue are also varieties of connective tissue. 

 Cartilaginous tissue is characterized by cells situated in a still firmer intercellular sub- 

 stance; and when the intercellular substance becomes combined with salts of lime, 

 forming bone, we have bony tissue. 



The blood-corpuscles originate from the mesoderm as independent cells floating 

 in the circulating fluid, the blood cells being formed contemporaneously with the walls 

 of the vessels enclosing the blood. In the invertebrates the blood-cells are either 

 strikingly like the Amoeba in appearance, or are oval, but still capable of changing 

 their form. Thus blood-corpuscles arise like other tissues, except that they become 

 free. 



Muscular tissue is also composed of cells, which are at first nucleated and after- 

 ward lose their nuclei. From being at first oval, the cells finally become elongated 

 and unite together to form the fibrillse ; these unite with bundles forming muscular 

 fibres, which in the vertebrates unite to form muscles. Muscular fibrillse may be sim- 

 ple or striated. The contractility of muscles is due to the contractility of the proto- 

 plasm originating in the cells forming the fibrillse. 



Nervous tissue is made up of nerve-cells and fibres proceeding from them; the 

 former constituting the centres of nervous force, and usually massed together, forming 

 a ganglion or nerve-centre from which nerve-fibres pass to the periphery and extremi- 

 ties of the body, and serve as conductors of nerve-force. — (Packard's Zoology.) 



Oegans. 



Animals are, with plants, called organisms, because they have organs. An organ 

 is any part of the body specially developed to perform a special kind of work. Thus 

 the wings are organs of flight, the heart is the organ of circulation, the leg an organ 

 of locomotion. The tissues we have enumerated are combined to form organs. The 

 simplest kind of an organ is perhaps the nucleus of the Amoeba. There are creatui-es 

 lower than the Amoeba which have no organs. These are the Monera, in which no 

 nucleus or any other specialized part of the body has as yet been found. If we rise 

 in the scale of animal life to the monad, we find that it has an external appendage or 

 organ like a whip-lash. In the Infusoria the body is covered with cilia, which are the 

 only organs of locomotion in these animalcules. In the Hydra, the only external organs 

 are the tentacles, which are situated around the head, and seem to feel for and to seize 

 its prey. In the higher worms we have oar-like organs of locomotion, arranged in 

 pairs on each side of the body ; also gills, or external breathing organs. Molluscs 

 have a creeping organ, the under side of the body; they also have gills and other 

 external parts or organs. In the crustaceans and insects the number and variety of 

 foi-m of external organs, especially the legs, gills, feelers, and mouth-parts, are remark- 

 able, and they are highly speciaUzed. In the vertebrates, beginning with fishes and 

 ending with man, we have external organs of sight, hearing, and locomotion, such as 

 fins, hands, and legs. 



Of the internal organs of the body, the most important is the digestive cavity, 

 which is at first in the gastrula or early embryo of all many-celled animals, and in the 

 Hydra and other polyps simply a hollow in the body. As we ascend in the animal 

 series we can trace its gradual specialization, beginning with the lower worms, and as- 

 cending to the annelids, also in the sea-urchin and starfish. In the molluscs, Crustacea, 

 and insects, as well as vertebrates, the alimentary canal is divided, during growth, into 

 distinct portions {i. e., the throat, stomach, and intestine), each with separate functions 



