PERIPATUS. 355 



longing, at least the May-fly, to existing families, appeared 

 iji the Devonian period, it is reasonable to suppose that in- 

 sects must have inhabited the dry land of the Silurian 

 period. 



Insects are divided into four sub-divisions or sub-classes — 

 i.e., the Malacopoda, the Myriopoda, the Arachnida, and 

 the six-footed, winged insects, or Hexapoda. All agree in 

 breathing by air-tubes, and in having in most cases a tri- 

 regional arrangement of the segments of the body. By some 

 authors the three higher groups are regarded as independent 

 classes, but they are all modifications of a general insect 

 form, with similar mouth-jDarts, a single pair of antennae, 

 when these organs are present, with no palpus to the man- 

 dibles, and with other characters which render the class 

 Insecta equivalent to the class Crustacea. 



Sub-class 1. Malacopoda. — This group is represented by 

 a single animal, the strange Peripatus of troijical coun- 

 tries, in which the body is cylindrical, the integument, an- 

 tennae, and limbs soft, not chitinized, with the head not 

 separate from the body, and bearing a pair of many-jointed 

 extensible antennae, with two pairs of rudimentary jaws 

 (mandibles and maxillfe), and from fourteen to thirty-three 

 pairs of feet (according to the species). This animal differs 

 from other Arthropods in the fact that there are two widely 

 separated nervous cords sent backward from the brain ; 

 also in the minute, numerous tracheal twigs arising from 

 numerous minute oval openings (rudimentary spiracles) 

 situated irregularly along the median line of the ventral 

 surface of the body. The feet are soft, fleshy, and end in 

 two claws. Peripatus is viviparous. According to the 

 description and figures of Mr. Moseley, the young develop 

 much as in the chilopodous Myriopods {Oeop)hilus), show- 

 ing that Peripatus is nearer to the Myriopods than any 

 other group. That it is a tracheate animal was also proved 

 by Mr. Moseley ; but owing to the nature of the nervous 

 system and the minute tracheae and th-eir numerous irregular 

 spiracular openings, with no chitinous edge, this form cannot 

 be placed among the Myriopods. On the other hand, it has 

 some features recalling the Linguatulina and Tardigrada, 



