VASCULAR AND RESPIRATORY SYSTEMS. 263 



coelome. One of the spaces forms the pericardium, or 

 chamber in which the heqrt lies. 



Vascular System. 



Within this non-muscular pericardium, and moored to it 

 by thin muscular strands, lies the six-sided heart, which 

 receives pure blood from the gills (via the pericardium) 

 and drives it to the body. 



The arterial system is well developed. Anteriorly, the 

 heart gives off a median artery to the eyes and antennules, 

 a pair of arteries to the antennje, and a pair to the digestive 

 gland. Posteriorly, there issues a single vessel, which at 

 once divides into a superior abdominal, running along the 

 dorsal surface, and a sternal which goes vertically through 

 the body. This sternal passes between the connectives 

 joining the 4th and 5th ventral ganglia, and then divides 

 into an anterior and posterior abdominal branch. All these 

 arteries are continued into capillaries. 



From the tissues the venous blood is gathered up in 

 channels, which are not sufficiently defined to be called veins. 

 It is collected in a ventral venous sinus, and passes into the 

 gills. Thence purified by exposure on the water-washed sur- 

 faces, it returns by six vessels on each side to the pericardium. 

 From this it enters the heart by six large and several smaller 

 apertures, which admit of entrance but not of exit. 



The blood contains amosboid cells, and the fluid or plasma 

 includes a respiratory pigment, hsemocyanin (bluish when 

 oxidised, colourless when deoxidised), and a lipochrome 

 pigment, called tetronerythrin. Both of these are common 

 in other Crustaceans. 



Respiratory System. 



Twenty gills — vascular outgrowths of the body wall — lie 

 on each side of the thorax, sheltered by the flaps of the 

 shield. A current of water from behind forwards is kept 

 up by the activity of the baling portion, or scaphognathite, 

 of the second maxilla. Venous blood enters the gills from 

 the ventral sinus, and purified blood leaves them by the six 

 channels leading to the pericardium. 



Observed superficially, the gills look somewhat like 

 feathers with plump barbs, but their structure is much more 



