ITS OUTER SKELETON. 57 



Neck. 



The neck is a narrow cylindrical tube, with a flexible wall 

 strengthened by eight plates, the cervical sclerites, two of which 

 are dorsal, two ventral, and four lateral. The dorsal sclerites 

 lie immediately behind the head (fig. 14) ; they are triangular, 

 and closely approximated to the middle line. The inferior 

 plates (fig. 27) resemble segments of chitinous hoops set trans- 

 versely, one behind the other, rather behind the dorsal sclerites, 

 and close behind the submentum. There are two lateral 

 scierites on each side of the neck (fig. 27), a lower squarish 

 one, which is set diagonally, nearly meeting its fellow across 

 the ventral surface, and an oblong piece, closely adherent to 

 the other, which extends forwards and upwards towards the 

 dorsal side. 



Thorax. 



The elements of the thoracic exoskeleton are simpler in the 

 Cockroach than in Insects of powerful flight, where adaptive 

 changes greatly obscure the primitive arrangement. There are 

 three segments, each defended by a dorsal plate (terguni) and a 

 ventral plate (sternum}. The sterna are often divided into 

 lateral halves. Of the three terga the first (pronotum) is the 

 largest ; it has a wide free edge on each side, projects forwards 

 over the neck, and when the head is retracted, covers this also, 

 its semi-circular fore-edge then forming the apparent head-end 

 of the animal. The two succeeding terga are of nearly equal 

 size, and each is much shorter than the pronotum, contrary to 

 the rule in winged Insects.* 



those hitherto accepted. He classifies the appendages as pre-oral (Insect-antenn?e) 

 and post-oral, and makes the following comparisons : 



HEXAPODA. ACERATA. 



( = Insecta + Myriopoda ?) 



(1) Antennae. 



(2) Mandibles. 



(3) Maxilla. 



(4) Labium. 



(5) 1st pair legs. 



(6) 2nd pair legs. 



(=Arachnida + Limulus.) 



Absent. 

 Chelicerse. 

 Pedipalpi. 

 1st pair legs. 

 2nd pair legs. 

 3rd pair legs. 



CRUSTACEA. 



Absent. 

 Antennules. 

 Antennae. 

 Mandibles. 

 1st Maxillae. 

 2nd Maxillae. 

 1st Maxillipeds. 



(7) 3rd pair legs. 4th pair legs. 



Pelseneer (Q. J. Micr. Sci., 1885), concludes that both pairs of antenna; are post- 

 oral in Apus, and probably in all other Crustacea. 



* Many Orthoptera, which seize their prey with the fore legs, have a veiy long 

 pronotum. 



