PISCES. 



991 



deucy towards the amphibious type, we find 

 at the same time that the branchial as well 

 as the osseous system manifests a most interest- 

 ing transitional structure between the plagios- 

 tomous and osseous fishes. We have next to 

 consider that part of the respiratory system 

 which is organized for breathing immediately 

 the atmospheric air, or the lungs ; for I do not 

 know how otherwise to designate, according 

 either to their physiological or morphological 

 relations, those organs which in the technical 

 language of the ichthyologist would be termed 

 the swimming or air-bladder. The trachea, or to 

 use the same technical and partial nomenclature, 

 the " ductus pneumaticus," is a wide short 

 membranous tube, as in the Perennibranchiate 

 Reptiles. The glottis (fig. 524, c,) opens 

 near the posterior part of a long rudimental 

 thyroid cartilage (f) : a few lines posterior to the 

 isthmus faucium the opposite end of the trachea 

 dilates into a membranous sac, which commu- 

 nicates by two large lateral apertures with the 

 lungs. These are widest at their anterior extre- 

 mities, and gradually decrease in diameter to 

 the cloaca, behind which they terminate each 



Fig. 525. 



Respiratory and circulatory apparatus of Lepidosiren 

 annectens, after Owen. 



a, auricle ; b. ventricle laid open to show the ter- 

 mination of the vena pulmonalis, in which a 

 black bristle is placed ; c, bulbus arteriosus laid 

 open ; d, pericardium ; e, vena cava abdomi- 

 nalis; f, vena pulmonalis; 1, 2, 3, 4, 5, 6, 

 branchial arteries of left side ; m, pulmonary 

 artery ; n, pulmonary vein. 



in an obtuse point (fig. 525). They are 

 lodged in the dorsal angle of the abdominal 

 cavity behind the kidneys, and are attached by 

 cellular tissue to all the surrounding parts, and 

 especially to the ribs, of which they bear the 

 impressions on their posterior surface. The an- 



terior part of each lung is divided into four or 

 five small lobes, behind which it takes on the 

 form of a simple compressed bag, and so con- 

 tinues to its posterior extremity. The parietes 

 of the lung present a moderate thickness 

 throughout, and the whole of the internal sur- 

 face is cellular, the cells having the same pro- 

 portional size and form as in the respiratory 

 portion of the lung of a serpent. The cells 

 are largest and most subdivided at the anterior 

 part of the lung, the livid colour of which, in 

 the specimen dissected " by Professor Owen, 

 " attested the great vascularity of the part." 



In tracing the arrangement of the circulatory 

 system of the Lepidosiren the same interme- 

 diate type of structure is most interestingly 

 conspicuous. The heart consists of an auri- 

 cle, (fig. 525, a,) ventricle, (b,) and a bul- 

 bus arteriosus. The vena cava (e) termi- 

 nates in the right side of the auricle ; it is 

 joined by the two superior cavae and by the 

 single large pulmonary vein ; this vein (J ) 

 does not, however, communicate with the sinus, 

 but passes along entire and adherent to the 

 inner surface of the vena cava as far as the 

 auriculo-ventricular aperture, where it empties 

 its contents into the ventricle by a distinct 

 orifice protected by a cartilaginous valvular 

 tubercle. It needed only that the pulmonary 

 vein should have been dilated before its termi- 

 nation, in order to have established a bi-auri- 

 cular structure of the heart, as in the Amphi- 

 bious Siren. The same functional advantage 

 is, however, thus secured to the Lepidosiren 

 with a maintenance of the simple dicselous 

 type of the heart of the fish ; this continuation 

 of the pulmonary vein preventing the admix- 

 ture of the respired with the venous blood, 

 until both have arrived in the ventricle. 



The ventricle (fig- 525, b) is extremely 

 small; its parietes are thick and reticularly 

 muscular; a small round orifice leads to the 

 bulbus arteriosus (c). This is formed by a 

 short spiral turn of the dilated aorta, which is 

 concealed under a simple continuous fibrous 

 coat. The area of this part of the vessel is 

 almost entirely occupied by two continuous 

 valvular projections or their processes, which 

 are attached by one edge to the internal sur- 

 face of the aorta, and have the opposite margin 

 projecting freely into the arterial cavity. 



The aorta in this remarkable species fulfils 

 at once the office of a systemic, a branchial, 

 and a pulmonary artery ; it distributes on each 

 side six vessels, (fig. 525, 1, 2, 3, 4, 5, 6,) 

 corresponding to the six branchial cartilaginous 

 arches. The mucous membrane is produced 

 into a branchial fringe on the convex side of 

 the first, fourth, fifth, and sixth branchial arches, 

 and the correspond ing arteries are minutely sub- 

 divided before they are continued to the dorsal 

 side of the pharynx ; these four pairs of vessels 

 are therefore true or functional branchial arte- 

 ries. The mucous membrane merely invests 

 with a simple fold the second and third bran- 

 chial arches; and the corresponding arterial 

 trunks undergo no subdivision as they wind 

 round them, but are continued entire (as in the 

 Amphiuma and Menopoma) to their termina- 



