OF PLANOCERA ELLIPTICA. 323 



I. Period of evolution. This period embraces the time which elapses between 

 the origin of the egg and the laying. The egrr originates like the organic cells. It 

 generates its own mass around a new centre of attraction until it becomes capable of 

 leading an independent life. During this period the egg is dependent upon the 

 mother, and has its growth. 



II. Period of preparation. Embraces the time which elapses from the laying of 

 the egg to the first manifestation of the embryo. The egg is full grown ; the vitelline 

 mass divides, works and kneads its substance, and prepares it for the future being. 

 During this period the egg is in a state of rest, but is no longer connected with its 

 parent. 



III. Period of the embryo. Embracing the time which elapses between the first 

 manifestation of the embryo and its escaping from the egg envelope. The vitelline 

 sphere is in motion and assumes various shapes. 



IV. Period of the larva. Lasts from the escaping of the embryo from the egg, 

 to its transformation into a mummy-like body. The young animal moves freelv 

 about in the surrounding medium, still assuming divers shapes. • 



V. Period of the chrysalis. It commences when the lively larva, losing its 

 polymorphic shape, becomes an immoveable body, simple and uniform in external 

 appearance. The end of this period is not known. 



XVI. CONCLUSION. 



^ 1. The investigations which I have traced upon Planarians have led me to their re- 

 moval from the class of Worms, where they had ranked hitherto, into the division of 

 moUusca, and more particularly into the class of Gasteropoda.* The embryological 

 grounds for my so doing are the following : 



The embryogeny of Gasteropod molluscs, and more particularly of Nudibranchiata, 

 has such a striking resemblance with that of the Planarians which I have examined, 

 that any one familiar with the subject will acknowledge its evidence. Thus the divi- 

 sion of the vitellus in Polycelis variabilis, as observed by me several years ago, although 

 not published yet, seems almost an exact copy of the same phenomenon in Acteon vi- 

 ridis of the coast of France : M'hen the yolk is divided into four .spheres, four smaller 

 ones will appear opposite, and then the latter will remain stationary whilst the former 

 will follow out the process of the division. 



In Planocera eUiptica the division of the yolk does not diifer apparently from the 

 same phenomenon in Acteon chloroticus of New England, and likewise in several spe- 

 cies of Eolis and Doris, as well as Triton. There is the most striking resemblance 



* Essai on the Classification of Nemertes and Planaria, Ac— Amer. Jour, of Sc. and Arts, 2d Series, si. 1851, 

 41, and Proc. Amer. Assoc. Adv. of Sc, third meeting held at New-Haven in August, 1850, "VVashington, 1851, 

 258, 



