256 CONTRIBUTIONS TO THE NATURAL HISTORY 



We should, therefore, not wonder at the fact, that the zoological descriptions of the 

 lower animals include so much more of structural details than the descriptions of any 

 other group of animals. The truth is, that here it is impossible to characterize genera 

 or even species without alluding to structural details, just as it is hardly possible to give 

 a sketch of their structure without indicating the principal varieties of form under 

 which these animals appear. And it is of great importance for the progress of zo- 

 ology in general, that this should be the case ; for, as the study of radiated animals has, 

 of late, excited more interest than before, it will have a beneficial influence upon the 

 other branches of zoology, in carrying over into all its departments the methods now 

 employed in the description of these lower types ; when the results of comparative 

 anatomy and embryology will be united to form one great picture of the whole animal 

 kingdom, as is already the case in the study of Radiata. 



It is really an unexpected circumstance, that the investigations of the lowest animals 

 should lead to the most appropriate and natural method of studying, describing, and de- 

 picting animals at large. But if we consider how, in these Radiata, and perhaps in Me- 

 dusae more than in any other group, the changes which they undergo during life are in- 

 timately connected both with their habits and changes of structure, it will be obvious 

 that the study of their metamorphoses is entirely inseparable from the study of their 

 structure, and that, again, this study is intimately connected with that of their habits ; so 

 that, in this class, the natural history of these animals is truly what it should be in all 

 classes, an illustration of their form, of their structure, and of their embryonic develop- 

 ment, as well as of their respective habits. It is highly desirable that this method of 

 studying animals should be universally introduced ; and as soon as it is done for those 

 classes which, from their peculiar structure, have left remains of their former existence 

 buried in the strata which constitute the crust of our globe, another point of view will 

 equally be connected with those other considerations, — I mean, the study of fossils ; 

 when those isolated doctrines of natural history, comparative anatomy and physiology, 

 embryology, and paleontology shall be viewed universally as mere subsidiary depart- 

 ments of one science, — that of the development of life. 



In Hippocrene the form is particularly striking, and there is hardly any type among 

 the naked-eyed Medusae in which it preserves a more permanent appearance ; for this 

 animal, during its motions, is less liable to extensive changes in its outline than any 

 other genus. 



The gelatinous disk which forms the main mass of the body is so much bent over 

 downwards, and its margin so extensively bent inwards, as almost to close the body, and 

 to give it nearly the form of a regular sphere. (Plate I.) The contraction of the disk 



