XI 



PHYLUM ARTHROPODA 



607 



All the appendages of the Scorpion are post-oral in position, and 

 the most anterior the chelicerae are probably best regarded as 

 corresponding to the anternae of the Crayfish, the equivalent of 

 the Crayfish's antennules and of the antennae of the Cockroach not 

 being present. The pedipalpi would then be the homologues of 

 the mandibles of the Insect and the Crustacean. 



CRAYFISH. 



Antennules. 

 Antennae. 

 Mandibles. 

 First maxillae. 

 Second maxillae. 

 First maxillip'edes. 

 Second maxillipedes. 

 Third maxillipedes. 



COCKROACH. 



Antennae. 

 Absent. 

 Mandibles. 

 First maxillae. 

 Second maxillae. 

 First legs. 

 Second legs. 

 Third legs. 



SCORPION. 



Absent. 

 Chelicerae. 

 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. 499, 

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

 elongated, wide, straight tube, with glandular walls, corresponding 

 to the chylific ventricle of the Insect. Opening into the mesenteron 

 are five pairs of narrow tubes (Figs. 498 and 499, hep. du.) leading 

 into the substance of a large glandular body, usually termed the 

 liver (hep.), though its hepatic functions are doubtful. The proc- 

 todceum ( prod.) is a short, narrow passage ; into it there open two 

 delicate tubes the Malpighian tubes (mal.) which act as the 

 organs of renal excretion. 



Circulatory organs. An elongated tubular heart (Fig. 498, 

 hrt.) lies in the prae-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 or anterior and posterior aortas (ant. art., 

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

 the various chambers. The anterior aorta (truncus arteriosus) soon 

 bifurcates to form a pair of vessels which embrace between them 

 the oesophagus, 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 



