MR. ST. GEORGE MIVART ON THE VERTEBRATE SKELETON. 377 
paraxial elements—that is, parts which embrace externally the pleuro-peritoneal cavity, 
or serially homologous with other parts which so embrace it. 
I venture to assert, on the other hand, that they are rather hypaxial elements; con- 
sequently I agree with Carus and Professor Owen in considering that they belong to a 
different category altogether from that-to which ribs and hæmal arches belong; while at 
‘the same time I differ from these authors, and agree with Professors Goodsir and Huxley, 
in taking them to be certainly portions of the true endo-skeleton. 
My reason for so considering them—for counting these hard parts as belonging to the 
internal laminæ of the ventral plates, or as belonging to those parts (of the undifferentiated 
ventral plates) which correspond to the internal laminæ of those portions of the ventral 
plates which are differentiated into internal and external laminæ—is the relation which 
they (4.e. the branchial arches) bear to the root-vessels of the arterial system (Plate 
LIII. figs. 3 & 20, v). 
The pericardium, with its serous lining (derivations from the primitive pleuro-peritoneal 
sack), is continued for some distance along the great vessels which leave the heart. These 
continuations probably indicate the primitively greater extent of the pleuro-peritoneal 
. cavity, and that parts embraced by such pleuro-peritoneal extension belong to the internal 
laminæ of the embryonic ventral plates. Moreover the parts which spring from and are 
directly continuous with the parts so embraced may fairly be reckoned (in the absence of 
any proof to the contrary) as belonging to the same category. I mean that if the roots, 
so embraced, of the great vessels belong to the inner portion of the ventral plates, 
then the parts of such vessels, just beyond the point to which such investment can be 
traced, also belong probably to the same portion of the ventral plates. 
This is the more probable as the pericardium is continued on to the large blood-vessels 
so as to “form tubular prolongations, which become gradually lost upon their external 
coats ***, | 
Now in osseous fishes the heart and aortie arches lie outside and extend along the 
external margins of these branchial arches (Plate LIII. fig. 20, v); and consequently : 
these arches must belong to the inner portion of the ventral plates, and therefore cannot 
correspond to ribs or other hard parts which embrace the pleuro-peritoneal cavity on its 
outer side. ^ ex 
Professor Goodsirt long ago called attention to the fact that “in Fish and Amphibia 
the heart and the branchial artery and its branches are situated below and external to 
the skeleton of the branchial apparatus." He further expresses his wonder “ that those 
who refer the branchial and pharyngeal arches to a splanchno-skeleton, have not adduced 
the external position of the hæmal axis to these arches as an argument in support of their 
opinion ;” but he rejects the argument himself as involving consequences (concerning the 
mandible &e.—points which will be considered further on) which he regards as fatal, but 
Which are not at all so in my eyes. edis 
I have been much strengthened in my confidence as to the correctness of this view by 
finding that my friend Mr. W. K. Parker has independently arrived at a similar resült, 
at least as far as regards the non-correspondence between branchial arches and ribs. 
* Quain and Sharpey's * Elements of Anatomy,’ 5th edit., 1848, vol. ii. p. 1101. T Loc. cit. p. 129. 
VOL. XXVII. | 9E 
