Chap. II. CLASSIFICATIONS OF ACALEPIIS. 129 



tlusoid elements, and if free Medaste born from Hydroids are inferior to the Dis- 

 copliorje proper, and these, again, inferior to the Ctenophora\ It is certainly a 

 most striking circumstance, that the only fossil free Acalephs known should be a 

 Discophorous Medusa, for it is the type Ave should naturally expect to follow 

 Hydroids in course of time, when it has once been ascertained that the earliest 

 representatives of all classes are either the lowest of their type, or embryonic in 

 their character or synthetic in the complication of their structure, as I have shown 

 in the first volume of this work (pp. 107-122). 



Some general remarks upon the geographical distribution of the Acalephs should 

 naturally find a place here ; but it is so indispensable to a true appreciation of 

 the mode of distribution of animals, that their types should be correctly referred 

 to their r-espective natural divisions, that, before considering the classification of 

 the Acalephs in its details down to the genera and sj^ecies, no accurate picture 

 of their geographical range and mutual relations in space can fiiirly be presented. 

 I must, therefore, postpone the consideration of this subject to another part of this 

 monograph, when, in addition to the information already collected, I shall be able 

 to avail myself of the investigations made )jy my son upon the Acalephs of the 

 Pacific coast of North America. It is a matter of great interest to me thus to 

 have the means of comparing critically the Acalephs of the temperate zone, not 

 only of the two sides of the Atlantic, but also of the Pacific, and to be able to 

 complete, in a measure, the statements of Brandt relating to the Discophorous 

 Medusa3 collected by Mertens, most of which were descriljed, after his death, from 

 the drawino;s made bv the naturalists of the Seniavin. 



SECTION IX. 



CLASSIFICATIONS OF ACALEPHS. 



The improvements in the classification of the Acalephs have heen the consequence 

 of a gi-adual and successive expansion of the boundaries of that clas.s, resulting 

 from the recognition of Acalephian characters in animals at first not suspected to 

 be at all related to them. The class, as such, has not been at once recognized 

 as a natural group, in consequence of the want of such a striking similarity of its 

 members, as is observed, for instance, among Birds or Insects ; but it has grown 

 by successive additions, forced, as it were, by internal evidence upon the notice of 

 naturalists, and slowly acquired, at long intervals, by laborious, successive steps. 

 However, the very character of this gradual progress renders the study of the 



VOL. ni. 17 



