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MORPJIOLOGV OF CORAL I'nl.VI'S 



107 



tremity, and the layer generally undergoes certain other modifications. 

 The whole structure suggests a larval sense-organ for the forward, 

 aboral pole, a similar organ occurring in the free-swimming larvae 

 of most of the higher groups of animals. It altogether disappears 

 when fixation takes plaee by this extremity, thus having only a larval 

 significance. 



On first extrusion the oral aperture of the larva is usually indeter- 

 minate, the stomodseum is non-functional, and the interior is more 

 or less filled with highly vacuolated tissue. Soon after liberation, 

 however, the middle part of the latter becomes disorganized, and 

 extrusions of yolk, zooxanthellse, and cell debris take place through 



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Fig. 14. — Diagrammatic figure illustrating the first stage of fission in a polyp 

 of Manicina. Compared with the stage represented in figure 13, the six pairs 

 of secondary mesenteries (11) have become complete, and an additional 

 cycle of twenty-four pairs, constituting the fourth order or third cycle, has 

 appeared. The stomoda;um has become divided into halves, each having 

 six pairs of mesenteries attached to it. The plane of division is entocoelic, 

 and the pairs of directives (d) are situated at the opposite extremities. 



the oral aperture, the larva becoming at the same time thinner walled 

 and more transparent. The larvse usually settle within from one to 

 three days after being set free, but the free-swimming stage may 

 continue for several weeks, though little or no development takes 

 place in the meantime. 



POST LARVAL DEVELOPMENT 



Either before or shortly after extrusion of the larva, the six pri- 

 mary pairs of mesenteries (protocnemes), constituting the first cycle, 

 make their appearance. The organs arise in bilateral pairs, in a 



