. Elements of Comparative Anatomy. VASCULAR SYSTEM OF ARTHROPODA. 283 So far as is yet knowu tlie circulatory system of Peripatus is represented by a " dorsal vessel/' just as in the Insecta, so that here we find the simplest characters as compared with the other Tracheata. In the middle line of the ventral surface of the dorsal vessel there is a row of clefts, and it appears to agree with that of the Myriapoda in extending along the whole body, while in the Insecta it is limited to the abdomen. In Insecta it is attached to the wall of the body, and sometimes even to the trachete (in the
. Elements of Comparative Anatomy. VASCULAR SYSTEM OF ARTHROPODA. 283 So far as is yet knowu tlie circulatory system of Peripatus is represented by a " dorsal vessel/' just as in the Insecta, so that here we find the simplest characters as compared with the other Tracheata. In the middle line of the ventral surface of the dorsal vessel there is a row of clefts, and it appears to agree with that of the Myriapoda in extending along the whole body, while in the Insecta it is limited to the abdomen. In Insecta it is attached to the wall of the body, and sometimes even to the trachete (in the larvte of the Muscidfe) by the "alee cordis'' (Fig. 146, m). In the larvae it is divided into separate chambers, which are often not very distinct on the outside, and which have, owing* partly to the arrangement of these muscles, and partly to the position of the cleft-like venous ostia, a metameric signification. The variation in the number of these chambers is not very great; in most there are eight of them; there are very seldom more, more frequently less than this. But these numbers still require a much more exact examination. The blood which is taken into the cardiac tube by the ostia is driven forwards by the systole of the chambers, and so passes from chamber to chamber, and from the most anterior of these into the aorta, where the pouch-like folds of the edges of the ostia function as valves, and prevent it from returning to the heart. The aorta (Fig. 146, a) is a direct continua- tion of the heart, which, as compared with the Myriapoda, has disappeared from the thoracic metameres. It runs straight forwards to the cerebrum, but its more intimate relations after this are not exactly known. It is uncertain whether the branching of its anterior end, wliich is seen in some Insects, is a general phtenomenon. In any case the blood very soon passes through a lacunar passage between the separate organs into regular currents; this may be easily observed in transparent insec
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