Xll] ACTACUS 208 



heart called the sternal artery. This artery goes directly down- 

 wards and passing between the commissures connecting the third 

 and fourth pairs of thoracic ganglia divides into anterior and pos- 

 terior branches which lie beneath the nerve-cord and give blood to 

 it (30, Fig. 73). From the heart an ophthalmic artery is given 

 off which runs forwards over the dorsal surface of the stomach and 

 eventually gives branches to the eyes and antennules. In addition 

 there proceed forwards from the heart two pairs of arteries, the 

 antennary and the hepatic. The first pair supply the antennae; 

 each gives off on its way a large branch to the stomach, called the 

 gastric artery. The hepatic arteries are more deeply situated, 

 they run forward and break up into branches which are distributed 

 to the tubes of the liver. The muscles surrounding the arteries, as 

 in Vertebrata (see p. 434), do not carry out rhythmical contractions, 

 but keep up a steady pressure on the blood, called tone. When 

 the arteries are traced they break up into branches which finally 

 open into the spaces of the haemocoele. 



So far no mechanism has been described which drives the blood 

 to the pericardium. It seems to find its way there by the constant 

 pressure from behind of the blood driven out from the heart. 



Reviewing the structure of the animal we see that the production 

 of chitin by the ectoderm leads to the jointing of the appendages 

 from which the name Arthropod is derived, leads further to the 

 loss of cilia and the consequent necessity of the peculiar respiratory 

 mechanism, and to the disappearance of the coelom and most 

 of the excretory organs, and consequently to the peculiar form of 

 the genital organs. In fact, it would be hard to name a character 

 of the Arthropoda as distinguished from the Annelida which is not 

 directly *r indirectly traceable to the existence of the chitinous 

 shell. 



Class I. CRUSTACEA. 



The Crustacea are with a few exceptions, such as the Wood-louse, 

 inhabitants of the water, and they breathe either through the 

 general surface of the body or by means of gills. They have or 

 are believed to have once possessed two pairs of antennae and these 

 as well as their other jointed limbs are typically biramous, that is, 

 they consist of a basal portion or protopodite bearing two prolonga- 

 tions, the endopodite and the exopodite. They have at least three 

 pairs of appendages converted into jaws. 



