640 



ZOOLOGY 



SECT. 



median lobe divided by a cleft ; this is termed the genital 

 operculum (op.) ; at its base is the opening of the genital duct. To 

 the sternum of the second segment of the pre-abdomen are attached 

 a pair of remarkable appendages of a comb-like shape -the pectines 

 (pect.)- each consisting of a stem, along the posterior margin of 

 which is a row of narrow processes, somewhat like the teeth of a 

 comb ; the function of these appendages is doubtful, but is probably 

 sensory. The remainder of the segments of the pre-abdomen, and 

 all those of the post-abdomen, are devoid of appendages. The 



sterna of the third, fourth, fifth, and sixth 

 segments of the pre-abdomen, which are very 

 broad, bear each a pair of oblique slits -the 

 stigmata (stig.) leading into the pulmonary 

 sacs. 



In the interior of the cephalothorax, over the 

 nervous system, is a cartilaginous plate the 

 endosternite (Fig. 545) which serves to give 

 attachment to muscles, and is comparable to 

 the cephalic apodeme of Apus (p. 520). 

 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 antennae 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. 



FIG. 545. Endosternite 

 of Scorpion. (After 

 Lankester.) 



CRAYFISH. 



Antennules. 

 Antennae. 

 Mandibles. 

 First maxillae. 

 Second maxillae. 

 First maxillipedes. 

 Second" maxillipedes. 

 Third maxillipedes. 



COCKROACH. 



Antennas. 

 Absent. 

 Mandibles. 

 First maxillae. 

 Second maxilla 1 . 

 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 plmnjnx ; 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. 547, 

 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 



