THE SERIAL RELATIONS OF INVERTEBRATA. 



581 



the other hand, the comparison of a Polyzoon with a larval 

 Lamellibranch or Gasteropod, or with a Pteropod, leaves no 

 doubt in ray mind that the Malacoscolices have the same rela- 

 tion to the Mollusca, as the THchoscolices to the Annelida. 



A fifth gradation is presented by the Tunicata and the 

 Enteropneusta, which constitute the Pharyngopnetjstal 

 Seeies. I do not regard the Enteropneusta as of distinctly 

 low T er organization than the Tunicata, but rather as a col- 

 lateral group; and I conceive it to be probable that some lower 

 forms, connecting the Enteropneusta and the Tunicata with 

 one another and with the THchoscolices, will yet be found. 

 However this may be, Appendicular ia presents a grade of 

 organization but little higher than that of the Polyzoa. 



A sixth gradation is represented by the Echhstodeemal 

 Seeies. Like the foregoing, this series at present stands 

 isolated, 1 no annectent forms between the Echinoderms and 

 higher or lower groups being known. On the ground of the 

 uniformity of character of the larvae of the Echinoderms, 

 however, there can be little doubt that, if ever such forms are 

 discovered, they will prove to be allied to the Gephyrea, the 

 Trichoscolices, and the Enteropneusta. 



Thus the study of the gradations of structure among the 

 Metazoa leads to the conclusion that they fall into six series, 

 which may be arranged in the following tabular shape : 



Series. 

 I. II. III. 



ZOOPHYTIC. EcHINODERMAL. PhARYNGOPNEUSTAL. 



Ccelenterata. Echinodermata. Enteropneusta. Tunicata. 



Porifera. 



Physemaria. 



IV. 



Malacozoic. 

 Mollusca. 



Malacoscolices. 



V. 



Annuloid. 

 Annelida. 



Trichoscolices. 



VI. 



Arthrozoic. 



Arthropoda. 

 Chcetognatha (?). 

 Nematoscolices. 



1 I say, at present, inasmuch as the characters of the nervous system sharp- 

 ly separate the most vermiform of the Echinoderms from the most Echinoderm- 

 like Gephyrea. 



