602 



ZOOLOGY 



SECT. 



sent the blastopore, though the endoderm is formed by direct 

 modification of the cells in the interior of the yolk. Stomodseum 

 and proctodseum are developed as invaginations of the surface 

 layer. The thickening of the blastoderm gives rise to a pair of 

 germinal bands in which rudiments of the segments soon become 

 recognisable. Larval membranes do not occur. 



In some of the Diplopoda there is a metamorphosis, such as 

 will shortly be described in the embryo Insect, and the larva 

 (Fig. 497, B) has a singular superficial resemblance to an Insect, 

 owing to the presence at first of only three pairs of appendages on 

 the anterior trunk region. 



Fossil remains of Myriapoda have been found in strata as far 

 back as the Devonian. The more ancient fossil forms are not 



FIG. 497. Two stages in the development of Strongylostoma, one of the Diplopoda. 

 A, early stage in the formation of the larva, which already exhibits distinct segments; 

 at. antennae. B, larva immediately after hatching. (From Balfour, after Metschnikoff.) 



capable of being grouped in the same orders as the living repre- 

 sentatives of the class, and are looked upon as constituting at 

 least two orders, the members of which are all extinct. While the 

 Progoneata, and, more especially, the Symphyla, show marked 

 resemblances to the Insecta more particularly to some of the 

 members of the order Aptera, the Opisthogoneata have features 

 connecting them through the Onychophora with the Annulata. 



CLASS IV.-INSECTA. 



The class of Insects (comprising the Cockroaches, Grasshoppers, 

 Dragon-flies, House-flies, Butterflies, Beetles and Bees, with their 

 many allies), though it is a very extensive one including as it does 

 a larger number of species than any of the other classes of the 

 Arthropoda is yet characterised by a remarkable degree of uni- 

 formity, no such extremes of modification occurring as are observable 

 within the class Crustacea. 



