We must repeat that such a system as we have just sketched forms a striking contrast to the imperfect blood-vascular system, and that Insects differ profoundly in these respects from Vertebrate animals. In the latter the blood-vessels penetrate to all {130}the tissues and form capillaries, while the aerating apparatus is confined to one part of the body; in Insects the blood-circulating system is very limited, and air is carried directly by complex vessels to all parts; thus the tracheal system is universally recognised as one of the most remarkable of the characters of Insects. Many Insects have a very active respiratory system, as is shown by the rapidity with which they are affected by agents like chloroform; but the exact manner in which the breathing is carried on is unknown. In living Insects rapid movements of contraction and expansion of parts of the body, chiefly the abdomen, may be observed, and these body contractions are sometimes accompanied by opening and shutting the spiracular orifices: it has been inferred that these phenomena are respiratory. Although such movements are not always present, it is possible that when they occur they may force the air onwards to the tissues, though this is by no means certain. It is clear that the tracheal system is the usual means of supplying the organisation with oxygen, but it appears to be improbable that it can also act as the agent for removing the carbonaceous products of tissue-changes. It has been thought possible that carbonic acid might reach the spiracles from the remote capillaries by a process of diffusion,[48] but it should be recollected that as some Insects have no tracheal system, there must exist some other mode of eliminating carbonic acid, and it is possible that this mode may continue to operate as an important agent of purification, even when the tracheal system is, as a bearer of air to the tissues, highly developed. Eisig[49] has suggested that the formation of chitin is an act of excretion; if so this is capable of relieving the system of carbonic acid to some extent. Others have maintained that transpiration takes place through the delicate portions of the integument. Lubbock[50] has shown that Melolontha larvae breathe "partly by means of their skin." The mode in which the carbon of tissue-change, and the nitrogen of inspiration are removed, is still obscure; but it appears probable that the views expressed by Réaumur, Lyonnet, and Lowne[51] as to inspiration and expiration may prove to be nearer the truth than those which are more widely current. In {131}connexion with this it should be recollected that the outer integument consists of chitin, and is cast and renewed several times during the life of the individual. Now as chitin consists largely of carbon and nitrogen, it is evident that the moulting must itself serve as a carbonaceous and nitrogenous excretion. If, as is suggested by Bataillon's researches,[52]