The respiratory membrane, in most cases, presents one of three forms, which have re ceived the names of gills or branchix, of tracheee, and of lungs. In the gills or branchim the tegumentary membrane is pro longed outwards, in the shape of laminm, tufts, or branches ; and this arrangement is found in aquatic animals. This form of the respiratory membrane is not, however, uni versal in thoe aquatic animals possessing special organs of respiration. In the Ho lothuridm, one of the tribes of the Echinoder mata, the chief respiratory organ consists of two aquiferous tubes, of an arborescent form, that open upon the surface of the cloaca.* In the Ascidians, among the Mollusca, the cbief respiratory organ is a large cavity, re garded by some as a dilatation of the ceso phagus ; and in certain of the aquatic Gastero poda it consists of a sac, with lamellm on its inner surface, opening upon the external sur face of the body. The small cavities placed along the sides of the body of the leech, and opening externally, are also believed to be respiratory organs.
The arrangement of the respiratory mem brane, termed trachem, is present in the Arti culata, among the Myriapods, insects, spiders, and also, with a few exceptions, among the larvm of insects living in the atmosphere ; and is observed in greatest perfection in the adult insects. It consists of a prolongation of the external membrane into the interior of the body, in the form of tubes, often ex tensively subdivided and ramified, kept open by fibres rolled round their walls in a spiral manner, and commencing at the external sur face of the body by orifices termed stigmata. In certain of the larvm of insects, this ar rangement of the respiratory membrane is modified to adapt it for aquatic respiration. In the larvm of the Ephemera, these trachem, instead of terminating in stigtnata, are prolonged outwards into a foliaceous expansion of the external integument, where they are subdi vided and ramified, and terminate in shut ex tremities. A constant interchange between the air in the trachem of these larvse, and the atmospheric air dissolved in the water, will go on through the membranes interposed between them.f In the larvm of the Libella the tracheae are distributed in a similar man ner in a membrane placed within the anus, and the animals draw in and expel the water with considerable force from that cavity, so that these respiratory movements act at the same time in causing locomotion.
The arrangement of the respiratory mem brane called lungs consists of the prolonga tion of the tegumentary membrane inwards in the form of sacs, and is destined for aerial respiration. In some of the terrestrial gas teropodous Mollusca, the lung is formed by a single, large, and simple sac, opening by an orifice on the right side of the body. In the Arachnida the lungs are composed of two or more separate cavities, lamellated on their in terior, opening on the external surface of the body, and are analogous to the branchial cavity in some of the aquatic Gasteropoda. In all the air-breathing Vertebrata, the respiratory membrane is formed by a prolongation of the internal tegumentary or mucous membrane from the upper part of the digestive tube, and this also holds in the aquatic Vertebrata, or the fishes. When the expanded respiratory membrane is placed at some distance from that portion of' the mucous membrane of the digestive tube with which it is continuous, as is especially the case in the Maramalia and birds, this mucous membrane is prolonged to the part where its expansion occurs, in the form of a tube, strengthened on the outer sur face by elastic textures to enable it to with stand the atmospheric pressure. Along this
tube (trachea), and its branches (bronchi and bronchial tubes), the air passes to and from the proper respiratory membrane on the inner surface of the lungs. In the water newt the lungs consist of a pair of elongated sacs, without any internal laminm or folds. In the frog these membranous sacs present ridges on their inner surface, especially at the upper part; and in the lungs of the turtle and crocodile these ridges increase in number and in size, and form partitions dividing the interior of the lungs into numerous cells communi cating with each other.
In birds the bronchial tubes on entering the lungs have numerous parietal cells on their inner surface; and this extension of the respiratory surface is still further increased by some of the trachem opening into membra nous bags, often presenting a cellular appear ance, and communicating with the interior of certain of the bones. In the lungs of man and the other Mammalia, the bronchial tubes divide and subdivide into minute branches, each of which ends in a cluster of terminal cells, forming one of the small lobes into which the lung may be divided. By this arrangement an immense extent of respiratory surface is packed up in a small space.* In those animals possessing special organs of respiration, this function is not necessarily restricted to these organs ; on the other hand, there are generally other parts of the organism which serve as auxiliary organs of respiration. We have already seen that re spiration may take place wherever the atmo spheric air and the nutritious juices are not separated by tissues impermeable to gases. When these tissues are feebly permeable by gases, or when the quantity of nutritious juices in these tissues is small, their re spiratory qualities must be feeble, however abundant the amount of atmospheric air in contact with them may be : while under more favourable physical conditions of the tissues, the aino tint of respiration effected may be con siderable. We can readily understand, there fore, how the external cutaneous surface in fishes and in the batrachian reptiles may con siderably assist the special organs of respira tion ; and how, in some fishes, the mucous surface of the digestive tube may act as an accessory organ of respiration when they rise to the surface of the water, and swallow a quan tity of air. As in the crocodiles, and in certain cartilaginous fishes, there are apertures by which the water may enter the peritoneal cavity, it is believed that in these animals the large ab dominal serous membrane, which is the chief respiratory organ in the Echinida, serves as an auxiliary organ of respiration.* In fishes the air-bladder, formed by a prolongation of the internal tegumentary membrane, and consti tuting a rudimentary lung, is generally consi dered to be an accessory respiratory organ. Even in the higher Mammalia, the external tegumentary membrane, and the internal tegu mentary membrane of the digestive tube, but more especially the former, may be regarded as auxiliary- organs of respiration, but the aid they afford to the special respiratory mem brane in the lungs is so feeble, that in a prac tical point of view they may in most cases be disregarded, and they can under no circum stances supply the place, even for a brief period of time, of the special respiratory membrane.