990 



PISCES. 

 Fig. 523. 



Lepidosiren. 



position of any contractile organs corresponding 

 with the left side of the heart of Mammalia, 

 immediately proceeds to give off. branches to 

 the trunk and abdominal viscera. Almost 

 close to its commencement it sends into the 

 abdomen the large visceral trunk, which sup- 

 plies all the abdominal viscera with blood, 

 being distributed to the liver, to the stomach, 

 to the intestines, to the spleen, to the generative 

 organs, and to the swimming bladder. 



After giving off the above large visceral ar- 

 tery, the aorta continues its course backwards 

 beneath the bodies of the supra-abdominal ver- 

 tebrae ; but as soon as it reaches the post-abdo- 

 minal vertebrae, it enters the canal formed by 

 their inferior arches (h<emapophi/se$), through 

 which it passes backwards to the tail. During 

 its passage through the abdomen the aorta fur- 

 nishes arteries right and left to the kidneys, 

 between which it lies; but with this exception 

 all its branches, which are given off opposite 

 each vertebra like the intercostal arteries of 

 the human subject, are distributed to the mus- 

 cles of the trunk. 



Venous system. All the arteries given off 

 from the aorta are accompanied by correspond- 

 ing venous branches, through which the blood 

 is returned into two large veins, one above and 

 the other beneath the vertebral column. The 

 former running with the spinal cord in the su- 

 perior vertebral canal, the other being in con- 

 tact with the aorta through its whole length, 

 these great veins frequently intercommunicate by 

 means of inosculating branches, and the inferior 

 one ultimately terminates in the great venous 

 sinus that opens into the auricle of the heart. 

 This sinus likewise receives through several 

 different trunks the veins from the liver, from 

 the generative organs, from the kidneys, from 

 the pectoral and vential fins, from the branchial 

 organs and other parts of the throat, and like- 

 wise from the head, the blood derived from the 

 head having been previously collected in a 

 large sinus. 



The highest form of the respiratory and cir- 

 culatory apparatus met with in Fishes is found 

 in the Lepidosiren, a remarkable genus found 

 in the river Amazon and in the Gambia.* In 

 this interesting creature the gills of the fish are 

 combined with rudimentary air-sacs which per- 

 form the office of lungs, and, in fact, the whole 

 arrangement of its circulatory system approxi- 

 mates so nearly that of the amphibious reptiles 

 that, were it not for its otherwise completely 

 icthyic characters, it might almost be regarded 



* Professor Owen, Description of Lepidosiren 

 annectens. Trans, of Linnaan Society, vol. xviii. 



as belonging to that order of Reptilia with 

 which it forms an interesting link of connec- 

 tion. The branchiae of the Lepidosiren consist 

 of separate elongated filaments attached by one 

 extremity to the branchial arches, which are 

 four in number. These cartilaginous branchial 

 arches are developed on each side in the sub- 

 mucous tissue, and are not attached either to the 

 os hyoides below or to the cranium above. The 

 membrane covering the third, fourth, and fifth 

 arches is minutely papillose, while the margins 

 of the first and second are finely denticulated, 

 and between these are five branchial apertures, 

 or interspaces, through which bristles are repre- 

 sented as passing in the accompanying figure, 

 (Jig. 524, 1, 2, 3, 4, 5.) 



The gills do not form 

 any external projection 

 as in the gill-bearing 

 Perennibranchian Am- 

 phibia, but are con- 

 tained in a moderately 

 capacious branchial 

 chamber, the parietes 

 of which are formed by 

 a mucous and muscu- 

 lar stratum, the external 

 outlet being a vertical 

 slit situated immedi- 

 ately anterior to the 

 filamentary pectoral 

 limb. 



" Thus," observes 

 Professor Owen, " al- 

 though the organs of re- 

 spiration through the 

 medium of water cor- 

 respond in all essential 

 points with those of the 

 true fishes, yet the gills 

 approximate in their 

 filamentary form to 

 those of the Perenni- 

 branchiate Reptiles. 

 And again, although 

 the gills are four in 

 number on each side, 

 as in the osseous fishes, 

 yet the number of 

 branchial apertures 

 and arches corresponds 

 with that which cha- 

 racterizes the higher 

 cartilaginous fishes. So 

 that, while we perceive, 

 even in the organs for 

 breathing water, a ten- 



Respiratory organs of 

 Lepidosiren annectens. 

 After Owen. 



a, first lobe of the 

 tongue; b, second lobe 

 of the tongue ; c, pha- 

 rynx ; d, pharyngeal 

 valve ; e, opening of 

 larynx ; /, laryngeal or 

 thyroid cartilage ; 1,2, 

 3, 4, 5, interspaces be- 

 tween branchial arches. 



