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musculature associated with the gill clefts. It also appears that if 
jaws were ever produced in connection with this branchial basket as 
jaws are produced from the visceral skeleton of Gnathostomes, they 
may, for all that the condition of the myotomes tells us, have de- 
generated and become transformed into the piston cartilage as AYERS 
and JACKSON and STOCKARD and some other authors suppose. Per- 
sonally I believe that the Petromyzont branchial basket cannot be 
homologized with Gnathostome visceral arches and, if this be the 
case, then of course true Gnathostome jaws can no more have existed 
in the ancestors of Petromyzonts than they have in the Myxinoids, 
but apart from this—a subject I shall refer to shortly—some other 
facts point to the same conclusions. Though the piston mechanisms 
of Petromyzonts and Myxinoids are not strictly homologous one with 
the other—another subject I shall refer to shortly—yet on the whole 
they are so similar that it might with reason be supposed that they 
arose in the same way and for the same purpose during phylogeny. 
This being the case, it needs a very powerful imagination indeed in 
order to assume that whilst the piston apparatus of Myxinoids arose 
as a special structure for a special purpose, the piston apparatus of 
Petromyzonts originated by the modification of a functional mandible.) 
Again the fact that the “lingual” (i. e. sub-branchial) myotomes of 
Petromyzonts retain the position of primitive myotomes at the sides 
of the head seems to point to the fact that jaws have never been 
present because in all Gnathostomes this same musculature has in 
every known case assumed a mid-ventral position between the rami 
of the mandible (fig. 4).2) It may of course be contended that when 
to the facts that in the latter the inhalent current can enter the gill-clefts 
via the hypophysial duct and that these animals do not attach themselves 
to foreign objects by the mouth (WoRrTHINGToN 40), whereas in the former 
the external gill apertures have to serve both for ingress and egress of water 
whilst the animals are attached by the mouth to stones or other objects. 
After writing the above suggestion I find that it has already been made by 
Ransom and THompson (32). 
1) Opponents may of course reply that the piston mechanisms of Pe- 
tromyzonts and Myxinoids are different from and non-homologous with each 
other because of this supposed difference in origin. I cheerfully leave to 
them the promising task of showing in what way the differences in structure 
of these two mechanisms are related to their assumed different modes of origin. 
2) I state this with diffidence, having no personal acquaintance with 
the subject and not being able in India to consult the papers of GEGENBAUR 
(13) and others. 
