CLASSIFICATION OF THE ANIMAL KINGDOM. 223' 



the generative and urinary orgTJns, it is obvious that it no less 

 closely answers to the " pleuroperitoneal "* chamber of the higher 

 Vertebrates. The opercular fold which constitutes the outer wall 

 of the branchial chamber in the Tadpole is formed by an out- 

 growth of the body-wall, as Kowalewsky states the wall of the 

 respiratory chamber in Amphioxus to be. On the other hand, in 

 all the higher Yertebrata, the somatopleure which bounds the 

 " pleuroperitoneal cavity " seems to be formed by a sort of split- 

 ting by the mesoblast, apparently very similar to the process which 

 gives rise to the perivisceral cavity of Annelida and Arthropoda. 

 And the discovery of the free communication of the great serous 

 cavities with the lymphatic system, has removed the objection 

 that might have been urged that the serous cavities of the Yerte- 

 brata are not parts of the vascular system. 



But it has been seen that it is only by the most careful study 

 of development that the " enterocoelous " " perivisceral cavity" of 

 the Echinoderm has been shown to be morphologically distinct 

 from the " schizocoelous " "perivisceral cavity" of an Annelid ; 

 and I think it probable that renewed investigation will prove 

 that the " splitting of the mesoblast " in the Yertebrata repre- 

 sents the invagination of the epiblast in the Aseidian, and the 

 formation of an epicoele by outgrowth of a ridge in AmjjMoxus. 

 Provisionally, at any rate, this hypothesis may be adopted, and 

 the Yertebrata in general, as well as AtnpJiioxiis, ranked among 

 the Epicoela. 



The discovery of the true head, brain, and renal organs of 

 Amphioxus removes the chief supposed anomalies of the struc- 

 ture of this animal, and to so great an extent bridges over the 

 supposed hiatus between it and the Marsipobranchii, with which 

 the development of the latter shows it to be very closely related, 

 that I see no reason for separating it from the class Pisces, in 

 which, however, it may properly rank as the type of a distinct 

 order, which may be termed Entomocrania, in contradistinction 

 to the rest, in which, as in all the higher Vertebrates, the skull, 

 even in the embryonic state, exhibits no indication of its primitive 

 segmentation t, and which may be termed Holocrania. 



* More accurately " pericardio-pleiu'operitoueal " chamber, as the pericar- 

 dium is ouly part of it, and, indeed, is only incompletely shut off in the Eays and 

 Myxinoid fishes. 



t See the proof of tliis position in my Croouian Lecture, ' Proceedings of the 

 Eoyar Society; 1858. . _ 



