226 



ICHTHYOLOGY. 



sessing the power of extracting air from water by means of 

 its gills, does not (except at rare intervals) require to mount 

 to the surface for the performance of the vital act of respi- 

 ration ; but all cetaceous animals being furnished with lungs, 

 which cannot perform their functions except through an 

 immediate communication with the atmosphere, require 

 their bodies to be terminated by a horizontal expansion, the 

 action of which is the most efficient for an ascending course. 

 It is, however, difficult to account for the feet that so con- 

 siderable an organ as the swimming-bladder should have 

 been denied to so many species, not only of tlie more in- 

 dolent kinds, which dwell composedly at the bottom of the 

 waters, but to many others which yield to none of their 

 class in the ease and velocity of their movements. Its pre- 

 sence or absence does not even accord with the other con- 

 ditions; of organization ; for while it is wanting in the com- 

 mon Mackerel, it is found to occur in a closely allied 

 species, the Scomber pneumatophorus of Laroche. Weber 



has pointed oiit a remarkable connection between the air- Classifica- 

 bladder and the acoustic organs in the head. 'io"- 



We have already alluded to a singular peculiarity con- ^^^■^'•-^ 

 nected with the organization of certain fishes — we mean the 

 power of conveying electrical shocks. In Torpedos, the 

 apparatus consists of two organs, built up of membranous 

 tubes filled with mucous matter, divided by transverse 

 chambers closely set together, like the cells of honeycomb, 

 and disposed in two groups placed on each side of the 

 head. They receive enormous branches of nerves from 

 the fifth and eighth pair. In the Gymnotus the electric 

 organs are four, and occupy the under surface of the body 

 throughout its entire extent to a considerable thickness. It 

 is composed of parallel plates separated by thin layers of 

 mucilage. The effect of this natural galv.-mic pile will be 

 detailed in the course of the systematic portion of this article, 

 when we shall have occasion to mention the electric fishes 

 ill their proper place. 



CLASSIFICATION OF FISHJJS. 



A " natural arrangement" of fishes or of any other class of 

 animals implies an accurate knowledge of their whole struc- 

 ture, internal as well as external, by which they can be 

 grouped in the order of their affinities, placing those species 

 together which agree with one another in the greatest 

 number of important characters. Naturalists have gene- 

 rally come to the conclusion that the series of species is 

 not a linear one ; some have one portion of their fi-ames, 

 some another, specially organized for the part the species 

 has to play in the system of nature ; and so the affinities 

 branch off in several directions. The nervous system being 

 that by which an animal has perception of external objects, 

 and directs its motions accordingly, and which presides 

 moreover over those operations of organic life that are not 

 obedient to the will, would be one important basis of classi- 

 fication, but its various modifications of structure in fishes 

 are still too imperfectly known, and the labour and skill 

 necessary for the elucidation and discrimination of its parts 

 and their functions are such that, practically, it has hitherto 

 been as yet but little referred to in the arrangements of the 

 systematic ichthyologist. The same is true in a greater 

 or less degree of the other parts of the organism employed 

 in nutrition, respiration and circulation, secretion, genera- 

 tion, and development; most of the arrangements of fishes 

 that liave hitherto appeared being based on the organs of 

 locomotion and the external integument, the latter the 

 most variable certainly that could have been chosen, but 

 at the same time the least important. We have scaly, 

 partially scaly, and scaleless species in the same genus of 

 some particular groups. Agassiz has been labouring assi- 

 duously on embryonic development as a basis of anange- 

 ment ; but as far as we know, has not yet published a system 

 founded on his researches in this direction. All attempts 

 at classification of fishes which have hitherto been given to 

 the world, violate more or less anatomical affinities ; the 

 best on the whole that has been proposed, is that of Pro- 

 fessor Johannes MuUer, which we shall follow, adopting the 

 modifications of Professor Owen. It would be a great help 

 to the memory were the divisions of the class of equal rank 

 to approach to equality also in the number of species that 

 they embrace, but this cannot be ; though most probably the 

 disparity in the size of the groups would be considerably 

 lessened were we acquainted with all the species secluded 

 in the depths of the sea, and still unknown, as well as with 

 the forms of the extinct fishes. The Sa/amti/idroid Ganoids 

 which abounded in variety and number in another epoch 

 of the earth's history, have only a few existing representa- 

 tives ; and of the pala?ozoic fislies with soil skins that are 

 less likely to be preserved than the strongly cuirassed Ga- 



noids, we know absolutely nothing, not even that such were 

 then created. 



In his aiTangement of fishes, Miiller finds characters 

 which he considers to be of the higliest importance in the 

 vascular system. The heart of fishes is a venous or bran- 

 chial one, consisting of an auricle in which the veins ter- 

 minate, and a ventricle for transmitting that venous blood 

 to the gills where it is aerated and whence it circulates 

 through the body without the intervention of a special pro- 

 pelling organ Hkb the systematic side of the heart of a 

 higher animal ; but merely through the general contractile 

 power of the arteries. In most fishes there is a thick 

 muscular swelling of the commencement of the arterial 

 system close to the ventricle, and which, in fact, may be 

 called a third chamber of the heart. The blood is pre- 

 vented from regurgitating into the ventricle on the con- 

 traction of the bulb) by valves ; and the number of these 

 , valves, and the presence or absence of the thick muscular 

 coat of the bulb, furnish characters of groups so constant, that 

 Mailer says he is acquainted with no others, either anatomical 

 or zoological, which equal them in certainty. The Plagio- 

 sfomi, or cartilaginous fishes, as restricted by the removal of 

 several groups that were included among them by the older 

 ichthyologists, have t/iree or more longitudinal roics of valves 

 within the muscular bulb. A still greater number of valves 

 are present in the bulb of the existing Ganoids ; but in the 

 large group formed by the osseous fishes after the Ganoids 

 have been removed from them, two opposite valves are 

 placed at the origin of the bulb, and no more. This great 

 group Muller names Teleoslei, or perfect osseous fishes; and 

 he includes in it the six following orders mentioned in the 

 subjoined table, — Acanthopteri, Anacanthini, Pharyngo- 

 gnathi, Physostomi, Plectognatlii, and Lophobranchii. 

 The Cyclostomes of Cuvier, or the Suckers, want the 

 bulbus arteriosus, or thickened muscular tunic, but have 

 two valves at the origin of the branchial vessel like the 

 Teleostei; while the Lancelet (Ampliioxus) wants the heart 

 itself, the circulation being carried on by the muscularity 

 of the entire vascular system. This fish, therefore, he con- 

 siders as the type of a sub-class which he names Lepto- 

 cardii. Another sub-class, termed Dipnoi, includes fishes 

 which have scales, with both lungs and gills. In all, he 

 makes six sub-classes of fish — 1. Teleoslei ; 2.. Dipnoi ; 

 3. Ganoidei ; 4. Elasmobrandiii ; 5. 3I(nsipobranchii or 

 Cycloslomi ; 6. Leptocardii. In the subjoined modifica- 

 tion of his arrangement by Professor 0«en, these sub- 

 classes are not preserved, the class being sub-divided into 

 nine ordei-s, and again into sub-orders. A iew changes 

 have been made by us in the termination of the names of 



