L. Agassiz on the Echinodermata. 31 



types of as many families. It is therefore necessary to exclude 

 from it the Sipuncuh, cSic., which constitute the second order 

 of this class in the Animal Kingdom of Cuvier, and to place 

 them amongst the Vermes. Thus reduced the class is charac- 

 terised especially by the presence of retractile pedicles arranged 

 in rows between the vertical segments of the integument of 

 the body. On account of this peculiarity M. de Blainville 

 has changed the name of Echinodermata, which is not really 

 applicable to the Holothuria, into that of Cirrhodermata, al- 

 though the nature and functions of these moveable organs, as 

 well as their relations to the external integument, are too im- 

 perfectly known at present to justify him fully in giving them 

 that name. The name of Radiated, borrowed from Lamarck, 

 and restricted to the limits which science now assigns to this 

 class, seems therefore entitled to a preference. It has the advan- 

 tage of being simple, and that of involving no systematic idea. 



The most general character commonly assigned to the Echi- 

 nodermata is that all the parts of their bodies are disposed like 

 radii about a common centre. This character they possess in 

 common with the whole division of radiated animals. How- 

 ever, on closely examining this radiated disposition of the 

 parts, it is observable that in different genera the rays are 

 not always alike, and do not always tend to a centre of the 

 same nature. My first care has been therefore to discover the 

 general laws of configuration and organization in this class, and 

 to determine the analogies which the different regions of the 

 body bear to one another and to those of other animals, in order 

 thus to obtain an appropriate terminology for their description. 

 The regular radiated disposition of the parts in most of the 

 Radiata renders it difficult to fix such a terminology. I thought 

 it best therefore to begin with the study of the forms most re- 

 mote from the star type, (in which an anterior and posterior, 

 a superior and inferior, and consequently a right and a left re- 

 gion are naturally exhibited,) in order, if possible, by easy 

 gradations to trace the same relations in the most regular and 

 even in the spherical and star forms. If, for instance, we ex- 

 amine the disposition of the parts in the Spatangi, we see at 

 once that the form of their body, more or less elongated, is a 

 consequence of the mouth and the anus being placed towards 



