Page:The New International Encyclopædia 1st ed. v. 17.djvu/78

BESPIBATOKY SYSTEM. and air) and of getting rid of carbon dioxide gas and rertaiu other excreuientitious produets. In Verlcbrata these organs are either gills or lungs. Ill invertebrate's the skin always finutions, more or less, as a respiratory organ, and in addition various other organs subserve this function also. Whatever form or |)Osition the resi)iiatory organ may assume, it must be richly supi)lied with blood vessels so that the blood containing carbon di- o.xide may come in close relation with the sur- rounding medium. No organ may be spoken of as a respiratory organ which has not this vascu- lar apparatus. Specialized respiratory organs do not appear in the animal kingdom until the segmented worms are reached, and even in many of the segmented worms no specific respiratory apparatus is developed. The integument of lower forms and sometimes the wall of the in- testine, when this organ is present, perform the respiratory function; the integument is better able to do so, since it is the part that comes most intimately in contact with the surrounding media. In echinoderms there are no respiratory or- gans that are homologous throughout the phylum. In holothurians this function is performed by the 'respiratory tree,' a mass of delicate branched tubules which' open into the cloaca; by the oral tentacles; and by the entire body-wall. In the other forms various other organs perform this function. Jlollusks, as a rule, have one (in a few' eases more than one) pair of breathing organs, the gills, which are covered and protected by the mantle. In some forms this gill is wanting and in others it is functionally replaced by other and phj'logenetically newer organs. Blood flows into these gills and after taking in oxygen from the water it flows back again, first to the heart and then to all parts of the body. The lamelli- brancli gills appear double on account of the strong development of the two rows of branchial leaflets, and in some mollusks increased surface is obtained by a folding of the leaflets. fScaphopo- da and many Gastropoda do not possess true gills. In certain cases they are replaced by adaptive gills. The latter may be delicate leaflets form- ing a rosette around the anus ; or folds to the right and left in the mantle cavity; or cerata, the dorsal finger-like processes of Nudibranehiata. Some Pulmonata have become adapted for aerial breathing. In this case the mantle cavity is richly supplied with blood vessels in its dorsal wall and gains a respiratory aperture, the so- called 'lung.' IJespiration in all Crustacea takes place in the outer integument. In small Crustacea thcTe are no specially developed respiratory organs, and respiration is performed by the entire body sur- face. In the large Crustacea certain parts of the body are adapted to perform this function more actively than others. To fit such parts ■ of the body for a respiratory function folds of the integument arise and the soft skin of the folds is functional in respiration. When this fold is transformed into a hard shield, it serves to pro- tect the delicate breathing organs. The function of respiration is performed in most Crustacea by the thoracic limbs. Some of the crabs live more or less on land and are adapted to re- tain water in the branchial cavity and to breathe air directly. In the latter case the part of the branchial cavity that functions as a lung has its 62 RESPIRATORY SYSTEM. integument modified into branched tufts which project into the air cavity and are well provided with vascular organs. The respiratory organs of insects are seg- mentally arranged air-conducting tubules, the trachese, which connect with the e.xterior by means of segmental openings, the stigmata. These openings are often guarded by bristles or tufts to keep out foreign particles. Internally the tracheal tubes branch and subdivide again and again and penetrate all through the tissues of the body. The ti'aehese are lined by chitinous spirals to keep the tubes open. This chitin is continuous with the chitin of the exoskeleton and is shed with the latter. In Amphioxus more than the first half of the length of the alimentary tract is devoted to the purposes of respiration, since its walls are pro- vided with gill-slits. This may be called the respiratory part of the alimentary tract (prosen- teron) in contradistinction to the remainder, hinder portion, the digestive part. Gill-slits to the number of 100, more or less, are borne by it. The variability in number is due to the fact that gills continue to increase in number as the ani- mal grows older, new ones being formed posteri- orly. In Ammocetes, of the Cyelostomi, there is a muscular fold at the posterior end of the branchial part of the oesophagus. In Petromyzon that part of the cesophagus which contains the gills is entirely cut ofl' from the alimentary tract and the cut-oil" end of the latter grows forw-ard above the gill-portion to join the mouth cavity. Thus two canals pass backward from the mouth. From Elasmobranchii onward the gills are in close relation with the skeletal parts of the visceral arches. As a rule teleosts possess four gill- arches, but certain rudimentary gills on the mandibular and hyoid arches indicate that fishes formerly possessed more gill-arches than at pres- ent. Fishes take in water through the mouth, and by a constriction of the latter force it out through the gill-slits. During the process of breathing the gill-arches rise and fall. The Dipnoi, as the name indicates, breathe both by gills and by lungs. The lung-sac of the Dipnoi is an unpaired bilobed sac. Lungs. The air-bladder and lungs have a similar development and the latter has developed from the former. Both are outgrowths of the alimentaiy tract; the air-bladder is usually formed on the dorsal, and the lung on the ventral side, however. The point at which the air-blad- der arises from the oesophagus is not a fixed one, and the duct which connects the bladder with the oesophagus may remain open, may be reduced to a solid strand, or may eventually disappear altogether. In the last case the air-bladder pi'obably gets its contained gas from its own Avail. The air-bladder always lies above the alimentary tract. In a few cases it is paired. It may be transversely constricted to form several divisions, or cn>cal processes may be present. The internal surface of the air-bladder of fishes may be sjiongy, a condition similar to that of the lungs of Dipnoi and Amphibia. The lungs of some of the Dipnoi show posteriorly a paired condition, although anteriorly there is only one part. The lungs of certain Amphibi.a ( Menobranchus and Proteus) are lower in development than those of the Dip- noi in so far as their internal surface is smooth. In reptiles and in the other vertebrates the form of the lung is determined by that of the body.