117: 



SCAX11IX AVIAN I'lSHES. 



PISCES CYCLOSTOMI. 



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 

 lieen of indisputable inseine nature. We now approach 

 tlie 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 Avithout firm skeleton. 

 But morphology and ontology (the de\elopment 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 whicli the characters of the different 

 classes were combined or not yet differentiated. The 

 Cyclostomous order survives during the modern age in 

 such a i)osition. The [iiiints of resend:)lance between 



these fishes and the rest are indeed so man)' tliat 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 Cyclostoraes as vertebrates lies in the metamorphosis 

 jjointed out by A. Miller" in the transition from Pride 

 {Ammocaies) to Lampern (I'efromiizo)i). The trans- 

 formation is here as great in certain respects as that 

 undergone by many invertebrates, and is also acconi- 

 jdished, at least in part, by histolysis (dissolution) of 

 the organs to be transformed. 



The order contains two families, one of which, tliat 

 of the Myxine, never acquires any true (discoid) suc- 

 torial mouth, the name of CycJostomi {with round mouthy 

 — Avhich we retain in accordance with the rules of 

 zoological nomenclature — being hence less appropriate 

 than the later Marsipobranchii {with sai<ate ffiUs)'. 



fam p e t r m y z n I d j:. 



Seven (/ill-ojieuinf/s on each side of the forepart of the bodi/ (the throat). The nasat duet terminates ca'caltij 

 and in a developed state has its inner {posterior) end situated hetween the anterior part <f the notixhord and the 



(esophagus, under the former and (diove the latter. 



well as hy the following family, \\ith relation to the 



On account of the al)ove-mentioned intermediate 

 position occupied ])y the family of the Lampreys, as 



other classes of vertebrates, its study has possessed 



" Miillers ArcMv 1856, ].. 32:3. 



'' C. DOmeril, Zoologie Aunh/tiipie, Paris 1806, p. 100, 



■■ BONAI'AKTE, Sdavhiorvm Uilmla tuwhjtica, N. Ann. Sc. Nat., Bologna, Ann. I, loni. II (1838), p. 214, anil Systema Mthyotogiai, 

 il.id., Anil. II, toiii. IV (1840), [i. 277. 



