. Elements of comparative anatomy. Anatomy, Comparative. TEACHER OF ARTHEOPODA. 287 same number of them, and in some cases they are found on every segment of the body. Eich stigma is an ellipse-shaped cleft, sur- rounded by a circular thickening of the external chitinous skeleton, which can be opened or closed by valves. Special muscles close these clefts. Each trunk from the stigma is lost, sooner or later, in a tuft of smaller branches, from which finer branches arise, which surround the organs. The way in which these branch, as well as the length and strength of the branches, varies greatly
. Elements of comparative anatomy. Anatomy, Comparative. TEACHER OF ARTHEOPODA. 287 same number of them, and in some cases they are found on every segment of the body. Eich stigma is an ellipse-shaped cleft, sur- rounded by a circular thickening of the external chitinous skeleton, which can be opened or closed by valves. Special muscles close these clefts. Each trunk from the stigma is lost, sooner or later, in a tuft of smaller branches, from which finer branches arise, which surround the organs. The way in which these branch, as well as the length and strength of the branches, varies greatly. Separate tracheal trunks may unite with one another, and form a system of tubes which passes longitudinally along, or transversely across the body, and from which finer ramifications are given off. Owing to the distribution of these tracheae throughout the body there is a great difference between the respiratory characters of the Tracheata and of the Branchiata. The medium which is to be respired is distributed through the whole organism; it is not only the blood- fluid which everywhere bathes the tracheae that can exchange its gases, but in the tissues themselves respiration can be effected, for the tracheae are distributed to them, and, indeed, may even come into relation with their form-elements (cf. Fig. 144, tr). But this does not apply to all cases, for, when the tracheae are reduced, the respiratory regions are diminished in number and extent; in this way the diffuse respiration is localised. In these cases, as when there are branchiae, the blood has to seek out the respiratory organs. In this way the characters of the tracheae influence the circulation. In addition to their respiratory function, the system of tubes filled with air serves to diminish the specific gravity of the body, and is just as important in this relation to Insects during their aquatic stages, as to those Insects which rejoice in wings, and which are able by special arrangements to increase or dimi
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectanatomycomparative
