i34 ELEMENTARY BIOLOGY. [CHAP. 



thoracic ganglia, passes between their commissures and 

 divides into two branches, which run, backwards and for- 

 wards, between the ganglionic 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 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 i branchio-cardiac^ 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, which is therefore, like the 

 heart of Anodon, a systemic and not a branchial heart. But 

 whether the whole of the venous blood takes the same 



