VASCULAR SYSTEM. 



777 



arisen independently in evolution. Whether this view is 

 correct or not, it is worth noting that organs, which closely 

 resemble tracheae and appear to have the same function, 

 are found in certain terrestrial Isopods (p. 482) and in certain 

 Siphonophora (vol. i, pp. 142-43). Finally, in some of the 

 forms, usually degenerate both in their size and in the absence 

 of heart and in reduced segmentation from parasitism or from 

 some other cause, the tracheae disappear altogether and the re- 

 spiration is entirely cutaneous.* 



The heart is contained in a pericardium and consists of a 

 certain number of chambers, in spiders and Phalangids three, 

 in scorpions seven and in 

 Limulus eight, indicated 

 by the ostia. The circu- 

 lation is largely lacunar, 

 and, as is the rule in 

 Arthropods, the chief 

 spaces of the body are 

 haemocoelic. Neverthe- 

 less in Limulus and in 

 the sco rpions both 

 arteries and veins are 

 well developed, and these 

 animals have a far more 

 definite system of blood- 

 vessels than have any other Arthropod. In the first named 

 genus and to a lesser extent in the scorpions the great gang- 

 lionic mass and the nerves which issue from it are sunk in or 

 are invested by the arteries so that the nerves are described 

 as being in the arteries. In the same two groups the walls of 

 the pericardium are fibrous and complete, the blood returning to 

 its cavity by paired veins which come from the respiratory organs. 

 The blood before passing to the gill-books or lung-books is col- 

 lected in a large median ventral sinus and Lankester has shown 

 that the upper wall of this ventral sinus and the lower w r all of 

 the pericardium are connected by a series of paired, vertical 

 muscles, seven pairs in Scorpio and eight in Limulus. The con- 

 traction of these muscles brings about the enlargement of the 



FIG. 506. Heart and vascular trunks of Lycosa, 

 in lateral and dorsal view (after ClaparSde). P 

 Lungs ; C heart ; Ao aorta ; eyes. 



* A similar disappearance of specialized respiratory organs occurs in 

 the abranchiate Urodeles, v. vol. ii, p. 278. 



