65(; 



ZOOLOGY 



not being present. The pedipalpi would then be the homologues 

 of the mandibles of the Insect and the Crustacean. 



Crayfish. 



Antennules. 

 Antennte. 

 Mandibles. 

 First maxillse. 

 Second maxilke. 

 First maxillipedes. 

 Second maxillipedes. 

 Third maxillipedes. 



Cockroach. 



Antennae. 

 Absent. 

 Mandibles. 

 First maxillse. 

 Second maxilliB. 

 First legs. 

 Second legs. 

 Third legs. 



Scorpion. 



Absent. 

 CheliceriB. 

 Pedipalpi. 

 First legs. 

 Second legs. 

 Third legs. 

 Fourth legs. 



Digestive system. — -The narrow mouth leads into a large 

 chamber with elastic walls, the pharynx ; this is capable of being 

 greatly dilated by the action of a number of radiating bundles of 

 muscular fibres which run outwards from it to the walls of the 

 cephalothorax, the result of this being to cause suction through 

 the mouth, by which means the juices of the Scorpion's prey are 

 drawn in. A second dilatation, to which a narrow oesophagus 

 leads, receives the ducts of a pair of salivary glands (Fig. 537, 

 sal. gld.). Upon this follows the mesenteron (mesent.), which is 

 an elongated, wide, straight tube, with glandular walls corre- 

 sponding to the stomach of the Insect. Opening into the 

 mesenteron are five pairs of narrow tubes (Figs. 536 and 537, 

 hep. du.) leading into the substance of a large glandular body, 

 usually termed the liver {hep.), though its hepatic functions are 

 doubtful. Into the long narrow intestine, in the first segment of 

 the post-abdomen, open one or two pairs of delicate tubes — 

 the Malpighiaii tubes (mal.) — which act as the organs of renal 

 excretion. 



Circulatory organs. — An elongated tubular heart (Fig. 536, 

 hrt.) lies in the pre-abdomen enclosed in a pericardial sinus ; it is 

 divided internally into a series of eight chambers by transverse 

 partitions ; into each of these chambers the blood passes by a pair 

 of valvular apertures or ostia. The heart ends both in front and 

 behind in main arteries, the anterior and "posterior aortoi {ant. 

 art., post, art.) ; and a series of pairs of lateral arteries are 

 given off from the various chambers. The anterior aorta soon 

 bifurcates to form a pair of vessels which embrace between them 

 the (Esophagus, and meet below in a median ventral trunk which 

 runs backwards above the nerve-cord. The blood carried to the 

 various parts of the body by the arteries is gathered up into a 

 large ventral sinus from which it passes to the book-lungs. From 

 these it is carried by a series of veins to the pericardial sinus to 

 enter the heart through the ostia. 



