OF THE MARSIPOBRANCH FISHES. 
403 
That there is no mistake here I feel certain; my own dissection shows the same 
pair of nerves, namely, the small, proper, 2nd branch of each trigeminal.* 
Now in these Myxinoids we miss the 3rd, 4th, and 6th cranial nerves ; also the 
glosso-pharyngeal or 9th nerve as a separate nerve. 
In Pet)omyzon, also, Muller misses the 6th or “abducens” (II., plate 3, figs. 3-5); 
the “ upper maxillary branch” of the trigeminal is small and inconspicuous but quite 
as large as in Bdellostoma. This being the case we have an explanation of what has 
been a difficulty with us for years ; Professor Huxley! speaks of “ a singular anomaly ” 
in both Petromyzon and the Myxinoids, in that “ Both the second and third divisions 
of the trigeminal nerve pass through the subocular membrane, and therefore on the 
ventral side of the (subocular) arch ; ” but this error was corrected by him soon 
afterwards. 
That the arch and the membrane (fenestra) are both the exact counterparts of what 
we find in the various Batrachian larvae I cannot doubt; and there is no doubt now, 
of the existence of a 2nd branch to the trigeminal in Petromyzon; and, as we have 
just seen, such an upper branch does exist in Bdellostoma, between the huge 1st 
and 2nd branches. 
Summary and conclusion of Part I. (Myxinoids). 
In seeking for light upon the primordial condition of the Vertebrata one naturally 
looks to such forms as the Myxinoids, for in these types, even in the adult state, 
there are neither limbs nor vertebrae, and no distinction between head and body, 
except the rudiments in the head, of a cartilaginous skull, a continuous structure, 
not showing the least sign of secondary segmentation, and by far the greater part of 
which is in front of the notochord, or axis of the organism. But here our gradational 
work agrees with the developmentcd, for the continuous skull bars constantly arise 
before the secondary cartilaginous segments that are formed between the “ Myomeres ” 
behind the head. Evidently, therefore, the early “ Craniata ” grew supports to the 
enlarged and sub-divided front end of the neural axis long before anything beyond 
strong fibrous septa were developed between the muscular segments of the body. As 
for the linear growth, and the less or greater extension backwards of the main organs, 
—circulatory, respiratory, digestive, and uro-genital—that in the variation of the pri¬ 
mordial form was a thing to be determined by the life and surroundings of the type. 
“ Thereafter as they may be,” was the tentative idea in this case. 
Certainly in the Marsipobranchs and in their relations the larval “ Anura,” we have 
the most archaic “ Craniata ” now existing; in these the organs may be extended 
far backwards in a vermiform creature, as in these low Fishes, or kept well swung 
* See also Retzius, plates 1 and 2, and note to p. 42, in Part II. 
t See “On the Nature of the Cranio-facial Apparatus of Petromyzon" (Jour, of Anat. and Phys. 
vol. 10, p. 423, a paper to which I am exceedingly indebted in working out these types). 
