118 ESSENTIALS OF ZOOLOGY 



below the seventh, the ventral plate of which is enlarged to 

 form the lamina subgenitalis. This is the terminal ventral 

 plate of the female. In the male the terminal ventral plate 

 is the ninth, and it bears a pair of styles. The shape of the 

 tenth tergum is different in the two sexes, and it bears in both 

 the segmented appendages called cerci. The purpose of the 

 cerci is not understood. They are evidently homologous to 

 the forceps of the nearly allied earwigs, and are believed to 

 function as posterior tactile, perhaps olfactory, organs. 



The openings are the mouth, anus, and genital apertures. 

 In addition there are ten pairs of spiracles or stigmata, which 

 open into the tracheal respiratory system : eight on the first 

 eight abdominal segments, and two on the thorax. On the 

 thorax they are situated between the legs on each side and 

 between the first and second and the second and third 

 segments. 



INTERNAL STRUCTURE. Movement is produced by striated 

 muscles attached to the segments as in the lobster. 



The alimentary canal consists of a fore-gut or stomodeum 

 derived from ectoderm; a mid-gut or enteron of endoderm; 

 and a long hind-gut or proctodeum formed, like the stomo- 

 deum, of ectoderm. Both the stomodeum and the proctodeum 

 are lined by chitin. The mouth is armed by the appendages 

 already described. The food is placed in the mouth by their 

 action, and in the cavity of the mouth it comes under the 

 influence of the fluid poured out by the salivary glands. There 

 are two salivary glands and a salivary receptacle on each side 

 of the crop. The long ducts of the two salivary glands join 

 to form a single duct which gives off the duct leading to the 

 receptacle. The salivary ducts thus formed on each side 

 fuse to make a common duct, and this opens on the hypo- 

 pharynx of the posterior side (floor) of the mouth cavity. 

 The short oesophagus expands into the crop, a wide and 

 large portion of the alimentary canal in which the food is 

 stored. It narrows rapidly at its hinder end to open into 

 the small thick- walled gizzard, or proventriculus, which is pro- 

 vided internally with chitinous teeth and posteriorly with setae. 

 The food undergoes a preliminary digestion in the crop, and 

 is introduced as required into the gizzard, where it is masticated 



