100 ACALEPIIS IN GENERAL. Part I. 



animals. Besides the data furnished by the investigations ah'eady referred to from 

 p. 28 to p. 3G, I had most desirable facilities for tracing the embryonic changes 

 of a considerable number of Acalephs. Indeed, I have been able to investigate 

 the embryonic growth of all the types of the class, with the sole exception of the 

 Diphyidaj and Physophorida\ But Leuckart, Kcilliker, Vogt, Gegenl)aur, and Hux- 

 ley have published such full .accounts and exhausting researches upon these very 

 families, that little is now wanting to complete the anatomical and embryological 

 liistoiy of the whole class. At all events, our comparisons may now extend to 

 every type belonging to this class. And the anatomy and endiryology of the 

 other classes of Radiates — the Polyps and Echinoderms — are also sutficiently w^ell 

 known to enable us to institute comparisons between them and the Acalejjhs, and 

 to trace the differences which l^ear upon the limitation of their respective classes 

 and subordinate groups. In attempting these comparisons, it is, however, indis- 

 pensable to bear in mind the dift'erence there is between general and special 

 homologies. 



General homologies lead to the knowledge of the identity of such sj'stems 

 of organs as present special structural combinations, and are perhaps adapted to 

 diflerent functions. The extremities of Vertebrates afford a good example of this 

 kind of homologies ; the pectoral fins with the thoracic arch of a fish, the wing 

 of a Bird or that of a Bat, and the arms of Man, are identical organs, however 

 diflerent they may appear, between all of which general homologies may be traced. 

 Special homologies, on the contrary, indicate the correspondence of identical parts, 

 diflering only in their relative proportions and special adaptations. The diflerent 

 systems of teeth, characteristic of the diflerent genera and families of Mammalia, 

 afford good examples of special homologies ; and may be studied from the extensive 

 investigations of Professor Owen upon that suliject. Now, the more animals are 

 compared in all their structural details, as well as in their various kinds and 

 different degrees of relationship, the more distinctly does it appear that general 

 homologies are co-extensive with the jjranches of the animal kingdom, while special 

 homologies are circumscribed Avithin the limits of the classes ; or, in other words, 

 that all the classes of one and the same branch have identical systems of organs, 

 however different the organs themselves may be, while the representatives of one 

 and the same class only exhibit identical adaptations in the structure of their 

 organs. Such a distinction, as flir as it may be carried out, aftbrds, therefore, 

 a valualjle additional test in the delimitation of the classes of animals. 



What the types are, which should be referred to the class of Acalephs, will 

 already appear from Avhat has been stated in Section II., ji. 41, where I have 

 compared the diflfereut types of Radiates with one another. It remains, however, 

 for me to prove that the assertions there made are founded in nature ; or, in other 



