VASCULAR SYSTEM OF ARTHROPOD A. 281 



The larvae of the Thoracostraca have a simple cardiac tube with 

 two lateral ostia only, which so far resembles the arrangement 

 described above. A more complicated form is gradually derived 

 from this, which is developed along two lines. One of these is seen 

 in the Stomapoda, where the heart is elongated, and an anterior and 

 a posterior arterial trunk alone given off, although the number of 

 venous ostia is increased. As the anterior artery alone is branched, 

 and the posterior has a widely-open mouth, the arrangement which 

 obtains in the Arthrostraca is again repeated in these forms. Later 

 on, not only do the anterior and posterior arteries form a larger 

 number of branches, but a larger number of lateral arterial trunks 

 are given off from the heart itself. 



The second type is seen in the Schizopoda and Decapoda. 

 Although the heart is provided with several pair of ostia it is more 

 concentrated in form, and the lumen can no longer be seen to be 

 divided into successive chambers. The primitive segmentation 

 has given way to a more compact arrangement. This cha- 

 racter is seen even in the arrangement of its several clefts, for they do 

 not now follow by regular pairs, but are grouped in a different manner. 

 The heart of the larvae, however, arises as a thin-walled tube with 

 only one pair of clefts, and is continued forwards and backwards into 

 a simple vascular trunk. The anterior one divides into three 

 branches, which arise directly from the heart when the trunk 

 shortens ; the posterior trunk remains single. The heart is elon- 

 gated for a time only, or passes at once into a more compact form. 

 In both the Schizopoda and Decapoda it is placed in the posterior 

 portion of the cephalothorax. 



New portions are formed in the arterial system, while the whole 

 venous portion is represented by lacuna?. The vascular system of 

 the Schizopoda (Mysis) remains at this stage; the Decapoda pass 

 through the different Schizopod- stages during their development. 

 In the mature forms of the Macrourous Decapoda Ave find the mus- 

 cular cardiac tube (Fig. 145, c) surrounded by a well-developed 

 pericardial sinus (pcj } from which the blood passes into the heart 

 by three pairs of symmetrically-arranged clefts. Three anterior 

 arterial trunks, and one posterior trunk arise from the heart. The 

 anterior median one (av) runs to the cerebrum and eyes (o), without 

 branching much; the two lateral trunks (aa) give off branches to 

 the generative organs, liver, and antenna?. The arterial trunk from 

 the hinder end of the heart divides into two branches, which lie one 

 over the other, and which may even arise separately from the heart. 

 The dorsal one (op), which is bifurcated in the Brachyura, supplies the 

 muscles of the back and tail. The ventral branch (a) takes a down- 

 ward course at once, and divides into an anterior and a posterior 

 trunk, both of which give off most of their branches to the appen- 

 dages. There may be two smaller trunks in addition to the posterior 

 median one. The highly-developed capillary system gradually 

 parses into afferent canals (veins), which are at first collected into 

 several trunks on the ventral side, and are then united (/•) into a 



