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 

 insect larvae; it is again collected into the venous ostia, which lie 

 near the entrance into the heart. The separate passages in this 

 tract are sometimes so sharply mai'ked off, that in the appendages, 

 for instance, vascular spaces appear to be formed. 



As the alee cordis do not end directly on the wall of the heart 

 but in special cells on it, and at the same time unite to form a 

 network surrounding the heart, a cavity resembling a pericardial 

 sinus is formed below them. 



§ 219. 



The regular extension of the heart of the Myriapoda through- 

 out the whole length of the body, and the considerable increase in 

 the number of its chambers, shows that there is in them a closer 



Fig. 116. Heart of 

 Melolontha. aAr- 

 tery arising from the 

 most auterior cham- 

 ber, m Alaj cordis 

 (after Burmeister) . 



