xxiv THE ANIMAL KINGDOM. 



as Protozoa or sponges, or, as in ccelenterates, it may be carried on in the water- 

 vascular tubes of those animals, while in the so-called respiratory tree of echin- 

 oderms it may go on in company with the performance of other functions by the same 

 vessels. Respiration, however, is inclined to be more active in such finely subdivided 

 parts of the body as the tentacles of polyps, of worms, or any filamentous subdivisions 

 of any of the invertebrates; these parts, usually called gills, present in the aggregate a 

 broad respiratory surface. Into the hollows of these filamentous processes, which are 

 usually extensions of the body-walls, blood is driven through vessels, and the oxygen 

 in the water bathing the gills filters through the integument, and immediately gains 

 access to and mixes with the blood. The gills of the lower animals api^ear at first 

 sight as if distributed over the body in a wanton manner, appearing in some species on 

 the head, in others along the sides of the body, or in others on the tail alone ; but in 

 fact they always arise in such situations as are best adapted to the mode of life of the 

 creature. 



The gills of many of the lower animals afford an admirable instance of the econ- 

 omy of nature. The tentacles of polyps, polyzoans, bi'achiopods, and many true worms 

 serve also, as delicate tactile organs, for grasping and conveying food to the mouth, 

 and often for locomotion. The suckers or 'feet' of star-fish or sea-urchins also without 

 doubt help to perform the office of gills, for the luxuriously branched, beautifully-col- 

 ored tentacles of the sea-cucumber are simply modifications of the ambulacral feet. 



In the molluscs, especially the snails and cuttle-fish, the gills are in close relations 

 with the heart, so that in tlie cuttle-fish the auricles are called ' branchial hearts.' The 

 gills of crustaceans are attached either to the thoracic legs or are modified abdominal 

 feet, being broad, thin, leaf-like processes into which the blood is forced by the con- 

 tractions of the tubular heart. Respiration in the insects goes on all over the interior 

 of the body, the tracheal tubes distributing the air so that the blood becomes oxyge- 

 nated in eveiy part of the body, including the ends of all the appendages. The gills 

 of aquatic insects are in all cases filamentous or leaf-like expansions of the skin permea- 

 ted by trachete ; they are, therefore, not strictly homologous with the gills of crusta- 

 ceans or of worms. — (Packard's Zoology.) 



We now come to the respiratory organs of the vertebrates, which are in close rela- 

 tion to the digestive canal. Fii-st the gills : just behind the mouth are openings, called 

 branchial clefts, on the edges of which arise processes, the gills or branchite. Through- 

 out these gills are distributed minute arteries and veins, forming a network ; the gills 

 are bathed in water taken in through the mouth. In the .amphibians and lung-fishes, 

 (Dipnoi) lungs, which are outgrowths of the enteric canal, replace the swinmiing 

 bladder of the fishes, the air being now swallowed by the mouth and gaining access by 

 a special passage, the larynx, to highly sjiecialized organs of respiration, the lungs, 

 which arc situated in the thoracic cavity neai- the heart. 



Nerves and Sensation. 

 We ha\e seen that animals of comparatively complicated structure perform their 

 work in the animal economy without any nervous system whatever. In none of the 

 Protozoans, even the highest infusorians, h.ave true nerve-cells been j'et detected ; in 

 these animals the tissues are in an inchoate, non-specialized state. It is not until we 

 rise to the many-celled animals that we observe nerves and nerve-centres. It has been 

 only recently discovered that in many jelly-fish there is, for the first time in the ani- 

 mal series, a true nervous system, with definite nerve-centres or ganglia. In the 



