318 



ICHTHYOLOGY. 



Classifica- 

 tion — 

 Sirenoids. 



SIKENOIDS. 



Tho distinct species of this order, family, and genus are 

 known ; one an inhabitant of tlie River Amazon, whose 

 structure lias not been so i'uily investigated, and the L. cin- 

 neclans from tlie Gambia, which lias been minutely dis- 

 sected by several of the first comparative anatomists of the 

 day, and been the subject of much scientific controversy as 

 to its station in the animal scale, whether it ought to be 

 ranked with the reptiles or fishes. Professor Owen from 

 the first advocated its ichthyic constitution, and BischofF 

 maintained its reptilian character ; but Owen's reasoning 

 appears to us to be perfectly conclusive. In a jjreceding 

 page we have briefly noticed the habits of the Siluroid 

 Hassars {Culliclithi/s) in taking slielter in the muddy bot- 

 toms of marshes, and when these dry u|i, digging burrows 

 for themselves to a considerable depth. They can live long 

 out of the water and make overland journeys in quest of it ; 

 but if we understand the accounts given by travellers, they 

 can, in the event of being cut off from water, pass the dry 

 season in an inactive state in their burrows, iiom whence 

 they are dug out by the natives, who use them for food. 

 Some moisture will, doubtless, always remain about them in 

 their subterranean abodes, but still their condition until the 

 rains commence must be analogous to the hybernation of 

 animals in higher latitudes ; they enjoy but a torpid kind 

 of existence, and use no exhausting muscular exertion until 

 the refreshing water flowing in recalls them to an active 

 employment of all the organs they possess as fishes. The 

 conditions of existence of the Lepidosiren annectans must, 

 as far as regards external circumstances, be similar during 

 the dry season, though that internal organization which ena- 

 bles it to pass much of its life out of water is of a very dif- 

 ferent kind. This fish inhabits the Gambia, which in the 

 rainy season floods extensive tracts of country. On the 

 waters retreating after the rains have ceased to fall, the 

 Lepidosirens that are left behind burrow into the mud, 

 which a tropical sun soon converts into a hard cake. An 

 aperture is left, however, that admits air, and they remain in 

 their chamber, torpid and clothed in a thick secretion of 

 mucus, until the water again overflowing the muddy lands, 

 releases them to the enjoyment of the swimming powers, 

 which they employ in search of food to allay the cravings 

 of a keen appetite. They have been brought to this coun- 

 try imbedded in clay round the roots of plants imported 

 from the Gambia, and their presence only became known 

 after the jilants were placed in the stove and well watered. 

 It is by a modification of the air-bladder rendering it ana- 

 logous in function to the lungs of an Amphibian, and also 

 by a peculiarity in the gills, that the Lepidosiren is adapted 

 to the alternate conditions of an active tadpole-like fish, 

 and an air-breathing dweller on the dry land. Asa general 

 nde, all fishes more highly organized than the Dermopteri 

 have, in their embryonic stage, five branchial arches and 

 five branchial arteries. In osseous fishes the anterior or 

 hyoid arterial arch developes either a gill of a single layer 

 only, or a rudimentary gill consisting of a network of ves- 

 sels, or both, or neither. The normal number of branchiae, 

 with double pectinated lamellie in osseous fishes is four ; — 

 most of the Labroids, the genera Coitus, Scorptfna, Sehas- 

 tes. Apistes, Zens, Cheiroiicctes, Gohiesox, Lepadogaster, 

 and Poli/pterus have three bilamellated or biserial gills and 

 one uniserial one ; Lop/iii/s, Batrachus, and the Chjmno- 

 donls have three biserial gills; and Molthcea and Lepidosiren 

 have two biserial gills and one uniserial gill. Two of the 

 arterial arches, which in other fishes supply a pair of bran- 

 chiae, remain undivided in Lepidosiren, and in their primitive 

 condition of vascular hoops, which end directly (in a man- 

 ner analogous to the ductus arteriosus of voung mammals) 

 in arterial trunks going to the air-bladders, and ramified 

 over their cellular surface. While passing throiigh the 

 ultimate twigs of these arteries, the blood is sufficiently oxy- 



genized to sustain life during the dry season, supplies of Classifica- 

 air being introduced into the air-bladders by a trachea whose tion — 

 (esophageal orifice is kept open by a laryngeal cartilage. Sirs'"""'. 

 Mr Owen sums up in the ibllowing words the particulars ^"^"v'^^ 

 of structure which induced him to consider the Lepidosiren 

 to be a fish : — " It may be presumed," he says, " that its ge- 

 latinous chorda dorsalis, its vertebral inferior transverse 

 processes (parapophyses), the normal attachment of the sca- 

 pula to the occiput, the branchiostegal covering of the per- 

 manent gills, the opercular bones, the absence of a pancreas, 

 the presence of a spiral intestinal valve, the relative position 

 of the anus, the extra-oral nasal sacs, the scaly integuments, 

 the nuicous tubes and pores on the head, the ' lateral line,' 

 and, in short, the totality of the organization of the Lepi- 

 dosiren, will be deemed fully to prove its true ichthyic na- 

 ture." This passage gives, in fewer words than we could 

 have employed, the reasons for introducing the order of 

 Sirenoids into our table of the class of fishes. In a discus- 

 sion which took place at a meeting of the British Associa- 

 tion soon after the publication of Mr Owen's paper on the 

 Le|)idosiren in the Transactions of the Linnean Society, 

 the debate turned much on the presence of a posterior nasal 

 orifice in the cavity of the mouth, but the Lepidosiren has 

 an elongated nasal sac, with the biserial pituitary folds usual 

 in fishes, and two apertures, neither of which communicate 

 with the mouth ; and Owen attributes to the position of the 

 nasal sac in the under part of the thick lip the mistake of 

 those naturalists who affirmed that the posterior opening 

 was within the mouth. We have in a preceding page men- 

 tioned an instance in the Si/nbranchidte of the posterior 

 nasal opening being as much on the interior side of the 

 lip as in Lepidosiren. Though the homology of the air- 

 bladder of Lepidosiren with the lungs of Batrachia is un- 

 deniable, and by a legitimate train of reasoning, the air- 

 bladders of other fishes are also proved to be homologous 

 with the lungs of the higher classes of vertebrals, there is 

 no ground, Mr Owen remarks, for asserting that the air- 

 bladders of fishes in general perform the functions of a lung. 

 Such oxygenation of the blood as a fish requires is eftijcted 

 through the agency of gills, though the relationship of these 

 organs to the tracheae and lungs of the higher vertebrals is 

 one of analogy merely. The branchiae of fishes are, how- 

 ever, homologous with the persistent or deciduous bran- 

 chiae of Batrachia, and with similar organs in embryos. 



In Lepidosiren the optic nerves do not cross each other, 

 agreeing in this respect with the same parts in Polypterus, 

 the Sturgeons, and the Plagiostomes. In ordinary osseous 

 fishes the nerves cross, but without an intermixture of ner- 

 vous fibrils. 



Order IX.— PKOTOPTERI, Owen. 



Sirenoidei, MUll. Endo-skeleton partly osseous, partly cartilagi- 

 nous; exo-skeleton as cycloid scales. Pectorals and ventrals as 

 flexible filaments. Gills filamentary, free. No pancreas; swim- 

 bladder as a double lung witli an air-duct ; intestine with a spiral 

 valve. 



Family I.— SIRENIDyE. 



Sirenoidei, Mull. General form resembling that of an Ophidium, 

 with a more tadpole-like tail. Pectorals and abdominal ventrals 

 single, slender, tapering barbels, supported internally by a soft 

 carUlaginous ray. Gills operculated. 



Genus I. Lepidosiren. (Protojyterus, Owen.) Two species, L. 

 annectans, Gambia; L.paradojca, Amazon. 



Order X.— HOLOCEPHALL 



Endo-skeletnn cartilaginous ; exo-skeleton as placoid granules. 

 Most of the fins with a strong spine for the first ray. Ventrals ab- 

 dominal. Gills laminated, .attached by their margins; a single 

 external gill-aperture. No swim-bladder; intestine with spiral 

 valve. Copula gaudent. 



Family I.— CHIM^RID.B. 



Cliimwroidei. Interiorly five gill-apertures which communicate 



