4 MARINE ANIMALS OF MASSACHUSETTS BAY. 



To the works above mentioned, we may add the third and 

 fourth volumes of Professor Agassiz's Contributions to the Nat- 

 ural History of the United States, which are entirely devoted to 

 the American Acalephs. 



The most important works and memoirs concerning the Echino- 

 derms are those by Klein, Link, Johannes Miiller, Jager. Des- 

 moulins, Troschel, Sars, Savigny, Forbes, Agassiz, and Liitken, 

 but excepting those of Forbes and Sars, few of these observations 

 are made upon the living specimens. It may be well to mention 

 here, for the benefit of those who care to know something more 

 of the literature of this subject in our own country, a number of 

 memoirs on the Radiates of our coasts, published by the varioiis 

 scientific societies of the United States, and to be found in their 

 annals. Such are the papers of Gould, Agassiz, Leidy, Stimpson, 

 McCrady, Clark, A. Agassiz, and Verrill. 



One additional word as to the manner in which the subjects 

 included in the following descriptions are arranged. We have 

 seen that Cuvier recognized the unity of plan in the structure 

 of the whole type of Radiates. All these animals have their 

 parts disposed around a common central axis, and diverging from 

 it toward the periphery. The idea of bilateral symmetry, or the 

 arrangement of parts on either side of a longitudinal axis, on 

 which all the higher animals are built, does not enter into their 

 structure, except in a very subordinate manner, hardly to be per- 

 ceived by any but the professional naturalist. This radiate struc- 

 ture being then common to the whole type, the animals compos- 

 ing it appear under three distinct structural expressions of the 

 general plan, and according to these differences are divided into 

 three classes, Polyps, Acalephs, and Echinoderms. With these 

 few preliminary remarks we may now take up in turn these dif- 

 erent groups, beginning with the lowest, or the Polyps.* 



* It is to be regretted that on account of the meagre representations of Polyps on 

 our coast, where the coral reefs, which include the most interesting featnres of Polyp 

 life, are entirely wanting, our account of these animals is necessarily deficient in vari- 

 ety of material. When we reach the Acalephs or Jelly-Fishes, in which the fauna of 

 our shores is especially rich, we shall not have the same apology for dulness ; and it 

 will be our own fault if our readers are not attracted by the many graceful forms to 

 which we shall then introduce them. 



