Chap. II. THE CLASSES OF RADIATA. 69 



apparatus provided Avith nerves and blood-vessels, and the seat of a special organ 

 of sense. In fiict, the Vertebrates alone have a real month ; and the opening leading 

 to the digestive cavity in other animals is in no way homologous to their mouth, 

 and ought to be called by another name, and by a different name in each type, 

 according to the general homologies of its structure. The so-called mouth of the 

 Articulates is as different from that of the Vertebrates, as it is from that of the 

 Mollusks and that of the Radiates. And if the name mouth is to be retained for 

 all, it must be with the distinct understanding that the mouth is essentially diffei'- 

 ent, both in its relations and in its structure, in Radiates, in Mollusks, in Articu- 

 lates, and in Vertebrates. I do not consider innovations in the nomenclature as 

 favorable to the progress of science, as long as it is possible to convey clear and 

 distinct ideas by the use of ordinary language ; but I believe, nevertheless, that 

 a new name, applied to an object long known under another appellation, impresses 

 more forcibly the difference it is intended to express, than a mere (j^ualification 

 of a generally received name. I would, therefore, propose to designate henceforth 

 the mouth of the Radiates by the name of Adinostome, that of the Articulates by 

 the name of Arthrostome, and that of the Mollusks by the name of Malacontomc, 

 in allusion to the typical structure of these animals. I shall introduce similar 

 changes in the nomenclature of other parts as often as, in the progress of my 

 exposition, I may have an opportunity of showing, not only the necessity of the 

 change, but also, by a fuller illustration of the homologies of these parts, the 

 propriety of adopting the new name proposed. 



The class of Echiiioderms is characterized by as different a mode of execution 

 of the plan of structure, involved in the idea of radiation, as the Acalephs and 

 Polyps are ; but the plan itself is the same in all. The peculiarity lies in the 

 construction only. The body-wall in Echinoderms forms a radiating cavity, in which 

 are suspended different systems of organs, distinct from the walls themselves, but 

 in various ways connected with them. The amlndacral system, which is homol- 

 ogous to the radiating tubes of the Acalephs and to the radiating chambers of 

 the Polyps, stands in the closest relation to the walls of the body. It traverses 

 them in the form of tubes, radiating from one pole of the body to the other, and 

 emitting, in most of them, external suckers, arranged in rows upon the surfoce. 

 The alimentary canal, connected with the walls of the body only at the central 

 opening, and, in some, also at the opposite end, extends as a distinct tube or sac, 

 free in the main cavity, and is not circumscribed by the perisome or spherosome^ 

 itself, as in Acalephs. The reproductive apparatus consists also of distinct organs, 



' I call spherosome the body-wall of a radiated of perisome because that of perisome has already 

 animal. I prefer the name of spherosome to that been applied with different meanings. 



