The different kinds of respiratory move ment may be arranged as follows ; — I. Info sorial Animalcules By projectile force.
2. Insects - - - Uncertain.
3. Fishes - - - By vacuum & jectile force.
4. Amphibia - - do. do.
5. Birds - - - do. do.
6. Mammalia - do. do.
Of the First Kind of Respiratory Movement.
Infusoria.— These animals breathe by a stream propelled in one direction, produced by the ra pid vibration of hair-like organs —cilia. (Vid. Art. CILIA.) Of the Second Species of Respiration. In breathing apparatus in insects generally reaches a high degree of develop ment. Sometimes respiratory tubes or tra chem penetrate every part, in the form of mi nute ramifying vessels conveying the included air to all the organs. The moving power or means of the renewal of the air ia these tubes is at present little known. Some insects, al though they live in water (as, for instance, the water beetles, and also water spiders), retain a bubble of air around them ; and, according to Nitzch*, they renew the air in the irtrachw by alternately elevating and depressing the an Beetles, before flying, seem to inflate themselves with air, so as to unfold their wings, which, like other parts of their body, are supplied with air tubes. In this case an inspirative effort must be made by some cavity. Under certain circumstances, bees emit a voice, a shrill sound, which is indepen dent of the motion of the wings, and which appears to be connected with the existence of a current of air through the respiratory tubes or trachea;; at least, such has been observed when the animal has been irritated and im mersed in water, the surface of which, where it was in contact with the orifice of the stig mata at the root of the wing, evidently vibrated at the moment the sound was produced.t In the orthoptera particularly, there are distinct respiratory movements, alternate dila tations and contractions of the abdomen ; in fact, respiratory motions are more distinctly perceptible in this division than in any other in sects. In the locusts verrucivora particularly, it is easy to distinguish how the abdominal rings, which have smaller abdominal scuta between them inferiorly, are alternately elevated and depressed exactly like ribs. If we smear the great thoracic stigmata with oil, we find that numerous bubbles of air escape from it during these motions.
The organs of respiratory motion, by means of which the supply of air is renewed, present many points of uncertainty. On the one
hand, where large stigmata are placed opposite to each other, and connected by tracheae, it is easy to see that alternate opening and shutting of their valves may produce a current capable of renewing the supply of air. It is conceiv able also, how, in the orthoptera, lepidoptera, and others, the expansion and contraction of the body, and the elasticity of the air-sacs contained in it, may cause the ingress of air. It is less obvious, however, how the same effect is produced in caterpillars and the larvm of beetles, where a current of air cannot very easily arise from the opposite position of the stigmata, on account of the minute ramifica tions of the trachea: ; and consequently we must look for some peculiar mechanism, pro bably cilia, unless we are disposed to admit the stagnation of the air in its vessels. IIence it has been conjectured that the dilatation and contraction of the dorsal vessel contributes to this purpose. This, however, appears to be scarcely possible ; and it might be asked on the contrary, if the vermicular motion of the body itself, the sliding of its segments upon each other, are not the means of keeping up the constant ingress and egress of air.* In the lowest of the molluscous class, the external tunic with which they are covered is generally so elastic, that it is capable of dilating by its own properties, when it has been greatly contracted by the muscular coat that is within ; and in forcible expirations, Dr. Grant has observed these animals to contract their muscular coat, and to retract the exterior covering, so as to propel, with considerable impetuosity, and to a distance, the water that fills their respiratory cavity. The elasticity of the tunic tends to over come the resistance of the muscular coat, and to expand, to a certain extent, the respi ratory cavity. Without, therefore, the existence of' elastic ligaments, such as we find in conchi fera, there is a partial means of enlarging the respiratory cavity given to these tunicated animals. This, however, is only in occa sional, forced, respiration ; constant and al ternate contraction and expansion of the ex terior tunic is not met with in any known tu nicated, nor in a conchiferous animal.-l- The streams that enter the respiratory, and pass out of the anal aperture, are smooth, regular, and incessant, and are produced by ciliary movement.