n PHYLUM ARTHROPOD A fi31 



forming a complete two-layered covering for it The outer layer 

 is termed serosa, the inner amnion. 1 



Each of the two mesoderm bands undergoes transverse division 

 into a series of segments, which become hollow and are then 

 closely applied to one another, eventually coalescing, so that the 

 cavities of all of them unite to form the coelome, the outer walls 

 becoming applied to the ectoderm to form a somatopleure, or lamina 

 consisting of somatic layer of mesoderm and of ectoderm ; the 

 inner being applied to the endoderm to form a splanchnopleure, or 

 lamina consisting of splanchnic layer of mesoderm and endoderm. 

 The body-cavity of the adult (hamiocaile) is not derived from the 

 coelome of the embryo. 



The ventral plate gradually grows upwards at the sides, and 

 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 develop- 

 ment 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, 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, 



1 This term is derived from one used in the Vertebrata, in which there is 

 an analogous membrane, occupying, however, a dorsal instead of a ventral 

 position as regards the body of the embryo. 



S S 2 



