618 



ZOOLOGY 



SECT. 



The sexes are always separate. There is usually an unpaired 

 gonad with paired ducts. In the Chilopoda the single genital 

 aperture is situated at the posterior end of the body: in the 

 Diplopoda and Pauropoda the two apertures are placed far forwards 

 towards the anterior end. 



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. 490. 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. 



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



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 proctodseum 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 

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

 (Fig. 490, B) has a singular superficial resemblance to an Insec 

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

 the anterior trunk region. 



Fossil remains of Myriapoda have been found in strata as fa 

 back as the Devonian. The more ancient fossil forms are no 



