Lectures on the Elements of Comparative Anatomy

XQL805H98 42

62 ON CJ..~ASSIFICATION. tion, these are either hard productions of tho alin1entary 1nucous me1nbrane, or are modified li1nbs. In the .latter case there may be 1nany pairs of them-numorou~ Orust~cea, for example, have eight pairs of limbs devoted to thJS function. In no verte­brated anima1, on the other hand, are litnbs so n1odified and functionally applied, the jaws being always parts of the cephalic parietes specially metamorphosed, and totally distinct in their nature from the limbs. All vertebrated animals, finally, possess a distinct vascular system, containing blood with suspended corpuscles of one kind, or of two, or even thr e, distinct kinds. In aJI, save one, there is a single valvular heart-the vessels of the exception, Amphiowus, possessing nun1 rous contractile dila­tations. All vertebrates possess a "hepatic portal sy. tem," the blood of the alimentary canal never being wholly returned directly to the heart Ly the ordinary veins, but l)eing more or less largely collected into a truuk, th "portal vein," whirh ramifies through and supplies the liver. These are the most important characters by which the verte­brate classes are distinguished, as a whole, from the other classes of the animal kingdmn; and their nn1nber and importance go a long way to justify the step taken by Latnarck wh 11 he di-ricled the animal kingdom into tho two primary subdivisions of VertP­brata and Invertebrata. If we seek now to construct definitions of the first two c]asseR of the Verteb1·ata, PISCES and Al\rPHIBIA, we ,'ha1l 1noet with some difficulties, arising partly from th wid ve:triutions ob­servable in the structure of fi hos, and partly frmn the (']osc affinity which exists between then1 and the Amphibia. No fish exhibits any trnce of that temporary appendage of the embryo of the higher verteLratos which is tenued an amnion, nor can any fish be said to possess an allantois, though the urinary bladder of fishes n1ay po sibly be a rudiment of that structure. The posterior vi. 'c ral clefts and arches* of fishes persist throughout life, and are usually more numerous than in othAr vertebrates ; while upon, or in connexion with, thCJn are * The relation of the perforated pharynx of A 1n7?hioxus to the vi::;ecml nrcltcs nnd clefts is not known. TilE AMPHIBIA. 63 developed villi, or la1nellm, which subserve the respiratory function. Median fins, formed by prolongations of the integument, supported by one or other kind of skeleton, are very character­istic of fishes, and it is questionable if any fish exists altogether devoid of the system of median fin-rays and their supports, which have been termed inter-spinous bones and cartilages. On the other hand, no vertebrate animal, other than a fish, is known to possess them. vVhen the limbs, or pectoral and ventral fins, of fishes are de­veloped, they always exhibit a more or less complete fringe of fin-rays. No amphibian is known to possess such rays in its lateral appendages, but there is some reason to believe that the extinct Ichthyosauria n1ay have been provided with them. In most fishes, the nasal sacs do not communicate dirertly with the cavity of the mouth, but the Mywinoids and Lepidosiren are exceptions to this rule. rrhe blood-corpuscles of fishes are always nucleated, and are commonly red, but by a singular exception those of Amphiowus (the Lancelet, which is an exception to most rules of piscine organization) are c.olourless. Almost all fishes have the heart divided into two auricles and one ventricle; but Amphiowus, as I have previously stated, is devoid of any special hQ, being provided instead with a number of contractile, vascular dilatations; while Lepidosiren possesses two auricles, and, at the same time, is provided with true lungs. It is useless therefore to appeal to the olfactory organ, the blood, the heart, or the respiratory organs, for characters at once universally applicable to, and diagnostic of, fishes. The AMPHIBIA (or Batrachians, Salamandroids, OrEcilim, and Labyrinthodonts) resemble fishes, and differ from all other verte­brates in the entire absence of an amnion, and in having only the urinary bladder to represent the allantois. They have red nucleated blood-corpuscles. Yet again they resemble fishes, and differ frOin all other vertebrates in the fact that filaments exer­cising a respiratory function, or branchire, are developed from their visceral arches during a longer or shorter period.