THE SERIAL EELATIONS OF INVERTEBRATA. 



581 



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

 Lamellibranch or Gasteropod, or with a Pteropod, leaves no 

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

 tion to the jHolluaca, as the IVichoscolices to the Annelida. 



A fifth gradation is presented by the Tunicata and the 

 Entcropneusta, which constitute the PHAEYNGOP^iTEUSTAL 

 Series. I do not regard the Enteropneusta as of distinctly 

 lower organization than the Tunicata^ but rather as a col- 

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

 forms, connecting the Enter opneusta and the Tunicata with 

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

 How^ever 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 Echixodermal 

 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 larvjB 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 Enter opneusta. 



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. 



n. 



Echijs^odeemal. 



Ccelenterata, Echinodermata. Enter opneusta, Tunicata, 



Porifera. 



Physemaria, 



I. 



ZOOPHYTIC. 



III. 



Pharyxgopneustal. 



IV. 



Malacozoic. 

 3follusca. 



Malacoscolices. 



Y. 



Ani^^uloid. 

 Annelida. 



Trichoscolices. 



VI. 



Arthrozoic. 



Arthropoda. 

 ChoBtognatha (?). 

 Nematoscolices. 



' I say, at present, masraucli as the characters of the nervous system sharp- 

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

 like Gephyrea. 



