XII] COCKROACH 237 



in Zoology, were suggested by fanciful and misleading comparisons 

 with the parts of the limb of a vertebrate. 



The abdomen consists of ten segments and here the terga and 

 sterna can be easily seen as they are not obscured by 



Abdomen. . . . * ? 



the insertion of the wings and legs. The eighth and 

 ninth terga are however both tucked under the seventh and are not 

 readily seen until the animal is artificially stretched.. The tergum 

 of -the tenth or last segment stands out from the hind end of the 

 animal and is cleft into two lobes. The sterna are equally distinct 

 but the first is small. The abdomen is broader in the female than 

 in the male, and the seventh sternum is shaped like the bow of a 

 boat and projects backwards, hiding the posterior sterna and sup- 

 porting the lower surface of a roomy pouch or cavity in which the 

 egg-case is formed. In the male the seventh sternum conceals the 

 eighth and ninth. 



The stigmata leading to the tracheae are placed in the soft 

 pleural membrane connecting terga and sterna. 



The abdomen is usually regarded as being without appendages, 

 but a pair of jointed cerci anales emerge below the edge of the 

 tenth tergum in each sex, and in the male the ninth sternum bears 

 a pair of anal styles. The claim of these structures to be 

 reckoned as appendages of the same rank as the antennae, the 

 gnathites and the legs, was at one time not generally conceded, 

 but appears to be now fairly well established for the cerci anales. 



In the soft tissue between the tenth tergum and the last visible 

 sternum, at the hind end of the body, is placed the anus, and below 

 it the single genital pore is situated. The anus is supported by 

 certain thickened plates in the skin, known as podical plates, 

 and around the genital orifices are arranged certain rods and bars, 

 symmetrical in the female but asymmetrical in the male, termed 

 gonapophyses, whose functions and meaning are obscure, but 

 which are connected with the processes of copulation and of egg- 

 laying. As we have seen, this region is in the female enlarged 

 into a genital sac by the growth and modification of the seventh 

 sternum and the tucking into the space so formed of the skin 

 carrying the eighth and ninth sterna. The opening of the oviduct 

 is on the eighth sternum and on the ninth is the single opening of 

 the receptaculum seminis or spermatheca, which consists of two 

 pouches of unequal size composed of inturned ectoderm : these are 

 always found full of spermatozoa in the fertilised female. The 

 spermatozoa apparently leave the spermathecae when the eggs are 



