xi PHYLUM ARTHROPODA 615 



eventually its borders meet and unite along the dorsal middle line, 

 the entire yolk thus becoming enclosed by it. 



The ventral nerve-chain is developed from a groove of the 

 ectoderm, bounded by thickenings which become detached from 

 the surface-ectoderm and form the chain of ganglia. The brain 

 is developed from a pair of ectodermal thickenings. That part 

 which is developed in the prostomial region the archicerebrum 

 becomes united with that developed in the following two segments 

 to form the completed brain or syncerebrum. 



It can hardly be said that the Cockroach undergoes a metamor- 

 phosis, the young Insect when it escapes from the egg differing 

 from the adult only in its smaller size and in the absence of wings, 

 which grow out subsequently from the terga of the meso- and 

 metathorax. Between its hatching and its complete development 

 the young Cockroach undergoes no fewer than seven " moults " 

 or ecdyses, in which all the chitinous parts become thrown off and 

 renewed. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Insecta are air-breathing Arthropoda in which the body 

 consists of three well-marked regions head, thorax, and abdomen ; 

 the head devoid of external segmentation, nearly always bearing 

 compound eyes, with a pair of antennae situated on the prostomium, 

 mandibles, and two pairs of maxillae ; the thorax of three segments 

 each bearing a pair of legs, and the second and third usually bearing 

 wings ; the abdomen composed of a varying number of segments 

 (7 11), which are devoid of appendages in the adult condition. 

 A liver is absent, but salivary glands are always present. There 

 is an elongated tubular heart, divided into eight chambers, situated 

 in the abdomen ; the vessels themselves are not highly developed. 

 The Insecta are, almost without exception, air-breathers, and the 

 organs of respiration take the form of branching tubes, the tracheae, 

 by means of which air is conveyed to all parts of the body. The 

 nervous system and sense-organs reach a high level of com- 

 plexity. The excretory organs are a number of blind tubes, the 

 Malpighian tubes, appended to the intestine. The sexes are 

 separate ; development is sometimes direct, more usually compli- 

 cated by metamorphosis. 



Sub-Class I. APTERYGOTA. 



Insecta which are completely devoid of wings and pass through 

 no metamorphosis. 



ORDER 1. THYSANURA. 



Wingless Insecta with an abdomen of ten segments, more or 

 fewer of which bear small appendages, and with two or three slender, 



R R 2 



