158 M. Agassiz on the Relations between Animals 



at once that I have arrived at the conclusion that the Forami- 

 nifera constitute the lowest type of Gasteropoda, and exemplify 

 under permanent forms the state of division of their germs in 

 their embryonic development. Thus circumscribed, the class of 

 Polypi constitutes a very natural group containing only animals 

 of an identical radiated structure, the organization of which is at 

 present very satisfactorily known. 



The class of Medusae has been from the beginning so well 

 characterized and circumscribed within so natural limits, that it 

 has undergone since its establishment only slight modifications 

 by the removal of some few genera : and after the position of the 

 so-called Hydroid Polypi among them shall have been generally 

 acknowledged, I believe it will undergo scarcely any new changes 

 in its extension, though we may still expect extensive improve- 

 ments, which are indeed very much needed, in the characteristics 

 and internal arrangement of their natural families. Considering 

 their structure, the Medusae rank immediately above Polypi. 



The Intestinal Worms have long been placed among Radiata, 

 and considered as a natural class in this great type of the animal 

 kingdom, notwithstanding so many striking differences in the 

 plan of their structure. This position was assigned to them upon 

 the ground of the radiated arrangement of parts around the head 

 and the vascular form of some of their genera, and also upon the 

 supposed want of a nervous system in all of them. But since the 

 discovery of nerves in all of their types, and since the most inti- 

 mate relations have been discovered between them and so many 

 other external woi'ms, their complete separation from theAnnelides 

 as a distinct class is hardly recognized now by any modern inves- 

 tigator. And the necessity of combining the intestinal parasitic 

 worms into one great natural group with the other external free 

 worms is becoming daily more evident to all, so that whatever 

 position be assigned to Annelides in the great type of Articulata, 

 Helminths have to follow them, and must therefore be removed 

 from the type of Radiata. This point is undisputed now, though 

 there may be a difference of opinion as to the propriety of ad- 

 mitting, to one great class, all Worms, or of subdividing them 

 into minor natural groups. 



The third class among Radiata is that of Echinoderms, which 

 has been circumscribed within most natural limits since the re- 

 union of Holothurise and Crinoids with the common star-fishes 

 and true Echini. Whoever is familiar with the embryonic deve- 

 lopment of Echinoderms, which has been extensively investigated 

 of late, will acknowledge an intimate relation between them and 

 the other two classes of Radiata, and not be willing to assent to 

 the proposed separation of Echinoderms as one great type in the 

 animal kingdom, placed upon an equal footing with MoUusca, 



