THE SERIAL RELATIONS OF INVERTEBKATA. 



581 



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

 Lamellibranch or Gasteropod, or with a Pteropod, leaves no 

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

 tion to the Mollusca^ as the Trichoscolices to the Annelida. 



A fifth gradation is presented by the Tmiicata and the 

 Enteropneiista, which constitute the Phaeyngopneustal 

 Series. I do not regard the Enteropneusta as of distinctly 

 lower organization than the Tu7iicata^ 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 Trichoscolices, will jet 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 Echinodermal 

 Series. Like the foregoing, this series at present stands 

 isolated,^ 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. ECHINODEEMAL. PhARTNGOPNEUSTAL. 



Ccelenterata. Echinodermata. Enteropneusta. Tmiicata. 



Porifera. 



Physemaria. 



» 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 Ge^hyrea. 



