1172 
SCANDINAVIAN FISHES. 
PISCES CYCLOSTOML 
Skeleton cartilaginous only at the head and tail, elsewhere fibrous. Notochord unaltered 
throughout life: no complete spinal column. No true maxillary or mandibular parts. No 
true gill-arches, the branchial lamellae being serially arranged on the inside of sacs each 
with a separate aperture or all having a common aperture on each side of the body. One 
nasal aperture only, in a developed state dorsal. No paired fins. No shoulder-girdle, no pelvis. 
Hitherto all the fishes under our consideration have 
been of indisputable piscine nature. We now approach 
the primordial limits of the vertebrate type, where the 
differences between the various classes find more or less 
imperfect expression in the structure of the several 
forms. Our knowledge of the geological evolution of 
the vertebrates is indeed deficient as yet in many re- 
spects, and fails us when we have to determine the 
origin of the classes. Besides, many remains cannot 
be expected to be preserved of animal forms which for 
the most part were indubitably without firm skeleton. 
But morphology and ontology (the development of the 
individual) point so clearly to intermediate forms which 
probably had representatives once in the living world, 
that the imagination has full right to picture an ex- 
tensive area of the system, principally tenanted by ex- 
tinct animals in which the characters of the different 
classes were combined or not yet differentiated. The 
Cyclostomous order survives during the modern age in 
such a position. The points of resemblance between 
these fishes and the rest are indeed so many that none 
can well deny the piscine nature of a Myxine or a 
Lamprey; but several portions of their structure lend 
themselves best to explanation by a comparison with 
the organs of batrachian larva?. 
A very salient proof of the low rank occupied by 
the Cyelostomes as vertebrates lies in the metamorphosis 
pointed out by A. Muller" in the transition from Pride 
(. Ammoccetes ) to Lampern (Petromyzon). The trans- 
formation is here as great in certain respects as that 
undergone by many invertebrates, and is also accom- 
plished, at least in part, by histolysis (dissolution) of 
the organs to be transformed. 
The order contains two families, one of which, that 
of the Myxine, never acquires any true (discoid) suc- 
torial mouth, the name of Cy cl ostomi (with round mouth) b 
— which we retain in accordance with the rules of 
zoological nomenclature — being hence less appropriate 
than the later Marsipobranchii ( with saccate gills) c . 
Fam. petromyzonidh. 
Seven gill-openings on each side of the forepart of the body (the throat). The nasal duct terminates ccecally 
and in a developed state lias its inner ( posterior ) end situated between the anterior part of the notochord and the 
(esophagus, under the former and above the latter. 
On account of the above-mentioned intermediate well as by the following family, with relation to the 
position occupied by the family of the Lampreys, as other classes of vertebrates, its study has possessed 
“ Mullers Archiv 1856, p. 323. 
b C. Dumeril, Zoologie Analytique, Paris 1806, p. 100. 
c Bonaparte, Selachiorum tabula analytica , N. Ann. Sc. Nat., BologDa, Ann. I, tom. II (1838), p. 214, and Systema Ichthyologies, 
ibid., Ann. II, tom. IV (1840), p. 277. 
