376 MANUAL OF ZOOLOGY. 



rudimentary external branchiae exist as well. The nasal sacs 

 open posteriorly into the throat. 



If these characters are examined a little more minutely, it is 

 easy to point to those in which the Lepidosiren approaches the 

 Fishes, and to those in which it resembles the Amphibians. It 

 resembles the Fishes in the shape of the body, and in the pos- 

 session of a covering of horny overlapping scales of the true 

 cycloid character; whilst the limbs are more like those of 

 fishes than of reptiles. The fin, also, which clothes the posterior 

 extremity of the body, is a decided fish-like character. The 

 most marked piscine feature, however, is the presence of free 

 branchiae, attached to branchial arches, and placed in a bran- 

 chial cavity, which opens internally into the pharynx by a 

 number of slits, and communicates externally with the outer 

 world by means of a single vertical gill-slit. 



P 



Fig. 144. Dipnoi. Lepidosiren annectens. 



On the other hand, the Lepidosiren approximates to the 

 Amphibians in the following important points : The heart 

 consists of three cavities, two auricles and a single ventricle. 

 True lungs are present with a trachea and glottis, returning 

 their blood to the heart by a distinct pulmonary vein, and in 

 every respect discharging the functions of the lungs of the 

 higher Vertebrates. It is true that the lungs of the Lepidosiren 

 are merely a modification of the swim-bladder of the other 

 fishes, but the significance of the change of function is not 

 aifected by this. Lastly, sometimes, at any rate, there are 

 rudimentary external branchiae placed on the side of the neck. 

 This feature, as will be seen shortly, is characteristic of all the 

 Amphibians, either permanently or in their immature state. 



Upon the whole, then, whilst for the purposes of systematic 

 classification the Lepidosiren must be placed amongst the 

 Fishes, it is not to be forgotten that many of its characters are 

 those of a higher class, and that it may justly be looked upon 

 as a connecting link, or transitional form, between the two great 

 divisions of the Fishes and the Amphibians. 



As regards their distribution and mode of life, two species at 

 least of Lepidosiren are known the L. paradoxa from the Ama- 



