II.] THE CRAYFISH AND LOBSTER. l8l 



branches, which run, backwards and forwards, between the 

 gangHonic chain and the exoskeleton. 



These arteries divide and subdivide and end in what, in 

 some parts of the body at any rate, e.g. the liver, is a true 

 capillary system. The veins are irregular channels, or 

 sinuses, which lie between the several muscles and viscera. 

 One of the largest of these is situated in the median ventral 

 line, and can be readily laid open by piercing the soft inte- 

 gument which lies between any two of the abdominal sterna. 

 The blood flows out of the aperture with great rapidity, and 

 the quantity shed shews the size of the sinus and its free 

 communication with the rest of the vascular system. By 

 cutting across any one of the limbs and inserting a blow- 

 pipe into the place whence the blood wells forth, this ventral 

 sinus can be readily injected with air. A large and irregular 

 sinus is also to be found in the median dorsal region of the 

 abdomen and is freely connected with the median ventral 

 sinus. The stem of each branchia contains two canals, one 

 running along its outer and the other along its inner face. 

 The outer canal communicates, at its origin, with the thoracic 

 portion of the median ventral sinus. The inner canal opens 

 into a passage which ascends in the lateral wall of the 

 thorax and opens, after meeting with other ' branchio-cardia? 

 canals, opposite the lateral aperture of the heart. As the 

 valvular lips of this and the other apertures of the heart 

 open inwards, the blood, when the systole takes place, is 

 driven out of the heart through the various arteries, and a 

 considerable part of the blood thus propelled into the 

 capillaries is collected by the median ventral sinus and 

 thence, passing through the gills, eventually returns to the 

 heart. It is customary to speak of a heart such as this, 

 which propels aerated blood, as systemic, by way of dis- 

 tinction from a branchial heart, which propels impure venous 



