of the Invertebrata. 223 



complete the series. Yet this arrangement would have the 

 apparent effect of crowning the order with the Crinoidea, 

 which, in the more common acceptation of the case, should 

 hold the earliest position. But the theory of the evolution of 

 new forms from preexisting ones would he evidently defective 

 if it did not admit of occasional retrogression in some points 

 of organization as well as progression in others, and even 

 often of considerable latitude of development of no certain 

 significance in either direction. 



4. Noctiluca has been chosen as the nucleus of the annulose 

 and articulate series, and would seem to be linked with the 

 Hotifera through such genera as Asplanchna, Gosse, and Asco- 

 morpha^ Perty. All things being considered, it is doubtful if 

 a more satisfactory selection could be made from the domain 

 of the Infusoria. 



From the Hotifera, through the Annelida, we may trace the 

 development of the crustaceous and chitinous types of Arti- 

 culata like a dichotomous branch. 



The Sagittidas no doubt hold an important place between 

 the Crustacea and the earlier annulose forms, in connexion 

 with which view their articulated jaws and finely striped 

 muscular fibres must be borne in mind. 



The first rudiment of a tracheal system is probably to be 

 sought for in the Terricolous Annelida ; but it is doubtful if 

 articulated limbs and a dorsal heart make their appearance 

 earlier than amongst the Julidas. 



Finally, a representative relationship has long been recog- 

 nized between the crustaceous Macrura, Anomura, and Bra- 

 chyura and the chitinous Myriopoda, Insecta, and Arachnida; 

 but however plausible the attempt may be to trace them back 

 to a more simple origin, it is certain that we can form no con- 

 ception of their further development. 



I here subjoin a tabular scheme of the leading divisions and 

 groups of the Invertebrata in accordance with the foregoing 

 views. 



Protozoa. 



Astomata. Stomatoda. 



Rhizo2)oda, Greciariniilcc. Infusoria. Noctilucidce. 



Ccelenterata. Cestoidea. Aproctaf. Rotifera. 



Molluscoida. Nematoidea*. Proctuchaf. Annelida. 



MoUusca proper. Trematoda. Echinodermata. Articulata. 



* Including Acantliopopliala. f Turbellaria. 



17* 



