STRUCTURE OF THE CRA YFJSH. 239 



more than a " liver," more even than a " hepatopancreas " ; 

 it is a " poly-enzymatic " gland, that is, one which produces 

 diverse digestive ferments. It makes glycogen like the liver, 

 and digestive juices comparable to those of the pancreas and 

 the stomach of higher animals. The hind-gut is long and 

 straight. It is lined by a chitinous cuticle, as its origin 

 suggests. There are a few minute glands on its walls. 



Body-Cavity. — The coelome is for the most part filled up 

 by the muscles and the organs, but there are interspaces left 

 which contain some fluid with amoeboid cells. Part of the 

 coelome also persists as the pericardium, or the chamber in 

 which the heart lies. 



Vascular System. — -Within this non-muscular pericardium, 

 and moored to it by thin, muscular strands, lies the six-sided 

 heart, which drives pure blood through the body, and 

 receives pure blood again {via the pericardium) from the gills. 



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 antennae, 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 runs vertically through 

 the body. The sternal passes through the connectives be- 

 tween the third and fourth 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 and aerated by exposure on 

 the water-washed surfaces, it returns by six vessels 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 amoeboid 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 skin — lie on each side of the thorax, sheltered by 



