of thf Invertehratd. 223 



coinj)lfte tlie scries. Yet this arrangement woukl have tlie 

 apparent etFect of crowning the order with the Crinoidea, 

 which, in the more common aceej)tation of the case, shouUl 

 hohl the earliest position. Hut the theory of the evohition of 

 new forms from ])reexisting ones wouhl be 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. Xoctiluca has been chosen as the nucleus of the annnlose 

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

 liotifera through such genera ns Asplanclinaj Gossq, and..l.sro- 

 morplid^ I'erty. All things being considered, it is doubtful if 

 a more satisfactory selection could be made from the domain 

 of the Infusoria. 



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

 devclo])ment of the crustaceous and chitinous types of Arti- 

 cidata like a dichotomous branch. 



The Sagittidie no doubt hold an important place between 

 the Cnistacea and the earlier annulose forms, in connexion 

 with which view their articulated jaws and finely striped 

 muscular fibres nuist be borne in mind. 



The first rudiment of a tracheal system is probably to be 

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

 articulated limbs and a dorsal heart make their a})pcarance 

 earlier than amongst the Julidre. 



Finally, a representative relationship has long been recog- 

 nized between the crustaceous ]\racrura, 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 tlie leading divisions and 

 groups of the Invertebrata in accordance w4th tlie foregoing 

 views. 



Protozoa. 



Astomata. Stomatoda. 



lihizopada. Grcgnrinida. Infi(soria. Noctilucidce. 



Ca4enterata. Cestoidea. Aproctaf. Kotifera. 



MoUuscoida. Nematoidea*. Proctucbat. Anuelida. 



MoUusca proper. Trematoda. Ecliinodermata. Articulata. 



* Including Acanthocephala. f Tuvbellaria. 



17* 



