THE SERIAL RELATIONS OF INVERTEBRATA. 581 



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

 Lamellibranoli or Gasteropod, or witb a Pteropod, leaves no 

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



t i< in t<> the M'tltuscd) as the Trichoscolices to the Annelid'. 



A fifth gradation is presented by the 7V/;/ />'//,/ and the 

 Enteropneitsta, which constitute the PIIARYNGOPM.I 

 SERIES. I do not regard the Enteropnemta as of dUtimtlv 

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

 lateral group; :ind I conceive it to be probable that some 1 

 forms, connecting the tint ro/meusta and the 'r>n>'><-t,, \\ith 

 one another and with the Tr/choscolice8,v?i\\ yet be found. 

 However this may be, Appendicularia 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, 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, und 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. 

 Codenterata. Echinodermata. Enteropneusta. Tunicata. 

 Porifera. 

 Physemaria. 



IV. V. VI. 



MALACOZOIC. ANNULOID. ARTHROZOIC. 



Mollusca. Annelida. Arthropoda. 



Chcetoffnatha (?). 

 Malacoscolices. TrlchoscoUces. 



1 I say, nt present, inasmuch ns the characters of the nervous systeni sharp- 

 y st'pnrnte the most vermiform of the Echino lerms from the most Echinoderm- 

 ikc (rehrea. 



