The Cockroach. 51 



wide sclerite above (the epicranium), divided by a Y-shaped 

 suture into right and left halves; each fork of the Y termi- 

 nates in a round soft spot of unknown function. In front of 

 and beldw this plate is the clypeus, with which articulates still 

 further forward the upper lip, or labrum — not an appendage. 

 At the sides of the head are two other pieces, the gence. 

 Above the gens are the conspicuous compound eyes, black in 

 colour, owing to the contained pigment. The thorax evidently 

 consists of three segments : each has a dorsal plate, the tergnm , 

 a ventral plate, the sternum ; and two smaller elements, the 

 episternum and epimerojt. From the mesonotum and metanotum, 

 as the terga of the second and third segments are called (the 

 first being pronotum), arise wings. The first pair of these are 

 stiff, and serve as covers (tegmina) for the second pair, which 

 are delicate, and folded when not in use. These wings are 

 processes of the terga, and may so far be compared to the 

 branchiostegite of the crayfish. The abdomen is made up of 

 certainly ten, and possibly eleven, segments ; the eleventh is a 

 pair of plates which lie on either side of the anus. Each 

 segment has a tergal and a sternal plate, connected by soft 

 membrane. The segments diifer somewhat in size, the eighth 

 and ninth being very small, and usually hidden by the seventh 

 in the female of £. oiientalis. Thus the entire body of Blatta 

 is made up of not more than seventeen segments. 



The antenna are not equivalent to the following appendages ; 

 they arise from the prostomium, and are therefore probably 

 the equivalents of cirri, which are found arising from the pros- 

 tomium in many aquatic worms. It is true that in the crayfish 

 both antennse and antennules are preoral in position; but a 

 consideration of the structure of the cerebral ganglia which 

 supplies them with nerves has led to the conclusion that it is 

 a compound structure containing not only the true cerebral 

 ganglia, but a pair of primitive postoral ganglia, which have 

 moved forwards. Thus, the first pair of appendages in Blatta 

 are the mandibles ; it is these that correspond to the antennules 

 of the crayfish. The mandibles are strong and jaw-like, consist- 

 ing of a basal piece only. The next appendage can be homo- 

 logized with the typical Crustacean limb, as already explained. 



