ON THE ORIGIN OP VERTEBRATES FROM ARACHNIDS. 355 



ment of powerful muscles of locomotor visceral arches or 

 Arachnoid appendages. 



The nature of the trabeculse^the occipital ring, the diaphragm- 

 like membrane, the absence of segmentation, and the relation 

 of the primordial cranium to the vertebrae are obscure points 

 which the anatomy and embryology of the Vertebrate head 

 failed to elucidate, but which the Arachnid theory resolves 

 into the comparatively simple question as to the origin of the 

 cartilaginous sternum. Vertebrate embryology, it seems, 

 told all there was to tell : the fault was not in the answer, but 

 in the interpretation ; or rather, in the conviction that if 

 ontogeny did not show what was expected, it was due to the 

 imperfections of the ontogenetic record, not of the expec- 

 tations. 



X. GiLL-SLiTs AND GiLL-ARCHES. — In Scorpio there are 

 coils of mesoderm (?) cells in the coxal portion of each pair of 

 thoracic appendages (Pis. XXIII, XXIV, figs. 1 — 3). All these 

 cell-coils disappear except that in the fifth coxa, which 

 develops into the adult nephvidium-like coxal gland ; hence 

 each of these coils probably represents a rudimentary nephri- 

 dium. 



An ectodermic invagination, which appears on the outer side 

 of the base of the fifth pair of legs, gives rise to the outlet of 

 the gland. There are similar invaginations at the bases of the 

 other appendages; but they give rise tochitin-lined tubes, 

 which serve for the support of muscles. The chitinised tubes 

 are comparable with the three or four pairs of tracheal invagi- 

 nations which in insects give rise to the tentorium. Since in 

 Acilius some of the abdominal tracheae at first communicate 

 with the cavities of the mesoblastic somites, it is probable that 

 all thetrachese represent the ectodermic portions of nephridia. 

 I regard the lung-books of Scorpio and the chitin-lined tubes 

 described above as belonging to the same category, for after 

 careful study I have found nothing to indicate that they arise 

 as modifications of rudimentary abdominal appendages. 



If we suppose that all the thoracic and vagus appendages are 



