m PHYLUM ARTHROPODA 623 



of which presents a deep notch ; below this is the opening of the 

 anus, at the sides of which are a pair of small hard plates — the 

 podical plates ; at the sides of the tergum are a pair of many- 

 jointed palp-like bodies — the cerci. The sternum of the first 

 abdominal segment is rudimentary. In the male that of the ninth 

 bears a pair of short styles. In the female the sternum of the 

 seventh is very much more prominent than in the male. The 

 genital aperture is situated on the ventral aspect of the posterior 

 extremity of the abdomen beneath the anal opening. 



When compared with the Crayfish, as regards the external 

 anatomy, the Cockroach is found to differ (1) in the arrangement 

 of the segments into regions ; (2) in the form and position of the 

 appendages. The head and thorax together correspond to the 

 cephalothorax of the Crayfish, but comprise fewer segments ; the 

 abdomen contains a larger number of segments. The single pair 

 of antennae probably correspond to the antennules of the Crayfish 

 — the antennae of the latter not being represented. On this view 

 the homologies of the anterior appendages in the two animals may 

 •be expressed in the following table : — 



Crayfish. Cockroach. 



Antennules. Antenna?. 



Antenna?.. Absent. 



Mandibles. Mandibles. 



First maxilla?. First maxilla?. 



Second maxilla?. 

 First maxillipedes. 

 Second maxillipedes. 

 Third maxillipedes. 



Second maxilla? (labium). 

 First legs. 

 Second legs. 

 Third legs. 



Representatives of the five pairs of thoracic legs of the Crayfish 

 would thus appear to be absent in the Cockroach, and evanescent 

 rudiments, no traces of which remain in the adult, alone represent 

 in the latter the well-developed abdominal appendages of the 

 former. 



In the living Cockroach respiratory movements are to be 

 observed, in which the abdomen becomes alternately expanded 

 and contracted ; these movements bring about the alternate 

 inhalation and exhalation of air through certain apertures — the 

 stigmata — at the sides of the body. Two of these are situated on 

 each side of the thorax, one between the prothorax and meso- 

 thorax, and the other between the mesothorax and the meta- 

 thorax. Eight occur on each side in the abdomen between the 

 terga and sterna of the segments. Just internal to each spiracle 

 the main air- tube or trachea into which it leads presents an 

 elastic ring or spiral, acting as a valve for closing the passage. 



