33© PHYLUM ARTHROPODA 



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 space between the gut and the body 

 wall is for the most part filled up by the muscles and the 

 organs, but there are interspaces left which contain a fluid 

 with amoeboid cells. These interspaces seem to represent 

 enlarged blood sinuses (a hasmocoele), rather than a true 

 body cavity or coelom. One of the spaces forms the blood- 

 containing pericardium, or chamber in which the heart lies. 



Vascular system. — Within this non-muscular peri- 

 cardium, 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 (ophthalmic) artery to the eyes and 

 antennules, a pair of (antennary) arteries to the antennae, 

 and a pair to the digestive gland (hepatic). 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 fourth 

 and fifth 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 surfaces, 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 amoeboid cells, and the fluid or 

 plasma includes a copper-containing respiratory pigment, 

 haemocyanin (bluish when oxidised, colourless when de- 

 oxidised), and a lipochrome pigment, called zoonerythrin. 

 Both of these are common in other Crustaceans. The 

 blood has a highly developed power of coagulation, so that 

 slight injuries do not lead to excessive loss. Sensitive 

 " explosive corpuscles " disintegrate readily when the 



