404 
MR. W. K. PARKER OK THE SKELETON 
beneath the head ; the body and tail, together, forming merely a propelling organ, as 
is seen in Tadpoles, especially the gigantic Tadpole of Pseudis. 
Thus we see that in low, limbless types there is no necessity for the development of 
more than fibrous metameres, but the vesicular brain, the suctorial lips, the purse¬ 
shaped gills, and the special organs of sense, these all call for support from some tissue 
more dense than a mere fibrous mat or web. In the Myxinoids we see that four 
special modifications of the connective tissue series are developed for the support of the 
properly cephalic organs, and two of these for them, only, so that these Fishes are 
chordate and craniate, but are not vertebrate, if we stick to the letter. At first some 
disappointment is felt, after careful study of these types, for notwithstanding the low 
level in which they remain, they are mere specialised Ammoccetes, keeping on the 
same “ platform ” as the larval Lamprey ; yet some parts of their organisation do undergo 
a marvellous amount of transformation, and are, indeed, as much specialised in conformity 
with their peculiar habits of life as any Vertebrates whatever, the highest not excepted. 
Yet, on the whole, the Myxinoids are a sort of Ammoccetine type, whilst the trans¬ 
formed Ammoccete, the Lamprey, comes nearest to the untransformed Frog or Toad,— 
the Tadpole. But the mere putting of this shows ( suggests at any rate) what losses the 
fauna of the world has sustained during the evolution of the “ Craniata.” For us, 
now, the Myxinoids, Petromyzoids and Anurous Amphibia must all be kept “within 
call” of each other ; but the types that have been cubed out, between them, cannot 
be numbered. 
Some other types of Fish are evidently the descendants of primordial “Marsipo- 
branchs,” notably Lepidosteus, the development of which has lately been made out 
and the results published in the Philosophical Transactions ( 1882 , Part II.). 
But the Chimceroids and Dipnoi, and, what is still more important, the Myxinoids 
themselves, have still to be followed through their early stages; if the present paper 
is of any value to the Morphologist, one on the embryology of these low forms would 
be worth many such papers. 
The Myxinoids keep on the low platform of the larval Lamprey (or Ammoccete) in 
the following particulars, namely :— 
a. The notochord has no paired cartilaginous vertebral rudiments in the spinal 
region. 
b. The trabeculae end in the ethmoidal region without growing forwards into a cornu 
(or two continuous cornua). 
c. There are merely “ barbels” round the mouth; no perfect labial cartilages. 
d. The last character involves another, namely, that the special armature of horny 
teeth attached to the labials in the adult Petromyzon is absent. 
e. The organs of vision are very feeble, and probably almost useless; in the 
Ammoccete they are arrested for a time. 
f. The cranium is a mere floor, without side-walls or roof. 
