This habit, which afforded a convenient way of collecting them, 

 I unfortunately did not discover until my last week at the ma- 

 rine laboratory. These larvae not only have the appearance 

 of planulae but their habits are the same. Agassiz' descrip- 

 tion of the habits of the planulae of Aurelia would apply very 

 well to these non-sexual larvae of Cassiopea. i 



The structure of the larva at this stage may be seen in a 

 longitudinal section, Fig. 29. The ectoderm is uniformly cil- 

 iated and consists of a rather deep layer of very narrow and 

 closely packed columnar cells. Their nuclei are small and are 

 arranged in several rows and the cells themselves are entirely 

 filled with a very opaque substance. Beneath the ectodenn is 

 a layer of supporting substance, containing a few green cells 

 and, occasionally, a colloblast. In a small area, at what was 

 the distal end of the bud, (D. Fig. 29) this layer is very thin; 

 there is a thicker zone, which diminishes in thickness towards 

 the equator, and in the proximal half of the animal the layer 

 is again quite tliin. In the distal end of the larva the four 

 septal muscles (s. m. ) are seen, each occupying a tube in the 

 jelly and beirg directly continuous with the ectoderm. This 

 is better shown in Fig. 28. The muscle fibres are already dif- 

 ferentiated and line the wall of the tube, while the nuclei are 

 more central. It is not usually possible to trace the muscles 



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