570 



ZOOLOGY 



SECT. 



the Diplopoda and Pauropoda the two apertures are usually 

 between the second and third segments of the body; in the 

 Symphyla the single aperture is situated between the legs of the 

 fourth segment. 



The ovum, as in most Arthropods, contains a large quantity of 

 food-yolk. The centrally-placed segmentation-nucleus divides so 

 as to give rise to a number of nuclei, this division being accom- 

 panied by a division of the yolk into a number of masses, which, 

 however, are more numerous than the nuclei. The nuclei then, for 

 the most part, migrate to the surface, some being left behind in 

 the yolk. Those that reach the surface, surrounded each by its 

 little clump of protoplasm, become arranged into a continuous 

 superficial layer of cells the blastoderm. On the surface of this 



FIG. 4/2. Two stages in the development of Strongylostoma, or.e of the Diplopoda. 

 A, early stage in the formation of the larva, whicn, however, exhibits distinct 



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



segments 



appears a thickening, and along the thickening is formed a groove 

 which may perhaps represent the blastopore, though the endoderm 

 is formed by direct modification of the cells in the interior of the 

 yolk. Stomodseum and proctodaaum are developed as invagina- 

 tions of the surface layer. The thickening of the blastoderm gives 

 rise to a germinal band 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 

 has shortly to be described in the embryo Insect, and the larva 

 (Fig. 452, B) has a singular superficial resemblance to an Insect, 

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

 on the anteriojitrunk region. 



Fossil rempns of Myriapoda have been found in strata as far 

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



