346 



PALAEOSTUACA. 



With the gradual diminution of the food-yolk and the develop- 

 ment of the dorsal region, the adult form becomes evident in the 

 embryo at the next stage. The anterior region of the body, which 

 consists of the cephalic segment and the six thoracic segments added 

 to it, has now the form of a shield, although its dorsal side is still 

 swollen and hemispherical. The dorsal side of this region of 

 the body now exhibits a segmentation corresponding to the six 

 thoracic segments; this is brought about by the re-distribution of 

 the food-yolk (rudiment of the enteron) and by the formation of the 

 mesodermal septa which grow inwards. On each side of the body, 

 six outgrowths of the yolk-laden entoderm (hepatic rudiments), 

 separated by mesodermal septa, can be recognised, the distal end 

 of the lobes being again secondarily branched (Fig. 158). The 



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f^ 



Fig. 159.— Two stages in the development of Limulus (after Kinosley, from Lang's Text-hook). 

 A, young embryo, just liatched in the Trilobite stage, seen from the dorsal side. B, embryo 

 a short time before hatching, seen from the ventral side. 



anterior lobes now no longer lie transversely, but radially. The 

 rudiments of the eyes have also become distinct. Those of 

 the middle eyes are originally situated ventrally (Packard, 'No. 2-3), 

 but soon sliift over the anterior margin of the cephalo-thorax on to 

 its dorsal surface. The rudiments of tlie lateral eyes correspond 

 in position (Watase) to the fourth hepatic lobe, and lie on the 

 inner side of the dorsal organ described by this author (Fig. 157). 

 The abdominal region now shows distinct segmentation, which, 

 however, according to Kingsley, only affects the internal organs, 

 while the ectoderm appears untouched by it (?). Nine abdominal 

 segments in all can be distinguished (Fig. 160), the last representing 

 the rudiment of the caudal spine, which is surrounded laterally by 



