TYPE TRAOHEATA. 487 



glandular sac which is probably to be regarded as a crural 

 gland. The last pair of appendages may be unjointed, each 

 bearing a tactile seta, and attached to the last segment is a 

 pair of conical processes each of which has opening at its tip 

 the duct of a spinning-gland. 



Two stigmata, situated at the base of the antennae, are the 

 only ones which occur, their position being very remarkable. 

 They open into bunches of branched tracheae which extend 

 throughout the greater portion of the body, leaving only the 

 appendages and the posterior part of the trunk destitute of 

 an air-supply. The Malpighian tubules attached to the 

 hind-gut are very long, and salivary glands opening upon the 

 maxillae are present. 



Eyes do not occur. The reproductive organs are situated 

 in the fourth trunk-segment and are paired. The oviducts 

 and vas deferens unite together and open to the exterior by a 

 single pore situated also upon the fourth segment, though it 

 has been described by some authors as situated at the pos- 

 terior extremity of the body. Very little is as yet known 

 concerning the development of Scolopendrella, whose similarity 

 to the Insects is shown in the antennee and the mouth-parts ; 

 indeed it has been advocated by some writers that the genus 

 should be associated with the Thysanura among the Insecta, 

 and the possibility of its being a connecting link between that 

 group and Myriapodous forms is indicated in the name applied 

 to the order. 



III. Class Insecta. 



The class Insecta is far richer in species than any other 

 class of animals, some two hundred thousand species belonging 

 to it being known to exist, and of these about eighty thousand 

 are beetles. A very large number are provided with organs 

 of flight and may be termed aerial ; others are terrestrial, 

 living either upon the surface of the earth or excavating bur- 

 rows beneath its surface ; while some have adapted them- 

 selves to an aquatic mode of life, and others are even marine, 

 members of the genus Halobates being found on the surface 

 of the ocean many miles from land. Many species, living as 



