METAZOA INSECTA. 393 



seven segments of the abdomen has a pair of similar 

 appendages (PI. 945, fig. 2), while the last segment bears 

 a pair of long appendages called cerci which resemble 

 the antennae in structure. 



The most characteristic system of internal organs in 

 insects is the tracheal system, but it is subject to great 

 variation owing to the widely different physical conditions 

 under which insects live. According to Meinert, Campo- 

 dea has three pairs of spiracles, one pair to each thoracic 

 segment, while the abdomen is without spiracles. In 

 Japyx, a closely allied genus, the abdomen has seven 

 pairs of these openings. The thoracic spiracles lead into 

 tracheae (colored blue in PI. 945, fig. 2) which do not 

 unite but simply form six small independent subsystems, 

 three on each side; of these the anterior subsystem gives 

 off branches to the head, and the posterior one to the for- 

 ward segments of the abdomen (see fig. 2). 



Bearing in mind that the general typical features of 

 Campodea represent the characters of the archetype of 

 the class Insecta,-we pass to some of its near allies. 



Lepisma saccharina Linn., or the " silver witch " (No. 

 948 ; PL 949, x 4, drawn from life), has a more flattened 

 body than Campodea and in this particular respect it is 

 probably nearer the ancestral form. Its body is not only 

 provided with hairs like that of Campodea, but many hairs 

 have become modified into delicate sculptured scales 

 (No. 950; PI. 95 1, fig, 5). 



The regions of the body are less uniform in breadth 

 than in Campodea, the abdominal segments tapering 

 away from the much broader thorax (see PL 949). The 

 head is free and of considerable size. 



The eyes of Lepisma are compound but they have only 

 twelve facets (Packard), so that they are primitive in 

 structure. 



A pair of extremely long antennae (see PL 949) are 

 attached to the head and a pair of maxillary palps extend 

 forward. The mouth parts in Lepisma are hinged to 



