216 VERTEBRATE ANIMALS. 



a ciliated pit, which is believed to be a rudimentary organ of 

 smell. 



OEDEK II. MARSIPOBEANCHII (Gr. marsipos, a pouch ; brag- 

 chia, gills). This order includes the Hag-fishes (Myxinidce) 

 and the Lampreys (PetromyzonidvB), and it is defined by the 

 following characters : The body is cylindrical and worm-like, 

 and is destitute of limbs. The skull is cartilaginous, there is 

 no lower jaw, and the notochord remains through life, so that 

 there is no vertebral column. The heart is composed of an 

 auricle and a ventricle, but there is no bulbus arteriosus. The 

 gills are pouch-like, communicating with the throat on the 

 one hand, and opening externally on the other by means of 

 apertures placed on the sides of the neck. 



The Hag-fish (Myxine) is an eel-like fish, which agrees 

 with the Lampreys in having neither pectoral nor ventral fins, 

 the representatives of the fore and hind limbs. The mouth is 

 of a very remarkable character, and enables the Hag-fish to 

 lead a very peculiar existence. It is generally found imbedded 

 in the interior of some large fish, into which it has penetrated 

 by means of a single serrated and recurved fang attached to 

 the centre of the palate. The mouth itself is destitute of 

 jaws, and forms a sucking disk or cup. Another remarkable 

 peculiarity of the Hag-fishes is found in the structure of the 

 nose. In all fishes, namely, except these and the Mud-fish 

 (Lepidosiren\ the nasal chambers are closed behind, and do 

 not communicate with the cavity of the mouth, as they do in 

 the higher Vertebrates. In the Myxinoids, however, such a 

 communication does exist. The nasal sacs are placed in com- 

 munication with the throat (pharynx) by means of a canal 

 which perforates the palate. A second canal leads from the 

 nasal cavities in front to open by an external aperture (the 

 nostril or "spiracle") on the top of the head behind the 

 mouth. 



Another peculiarity, which is best considered in the Lam- 

 preys, is to be found in the structure of the respiratory or- 

 gans, from which the name of the order is derived. ^ When 

 viewed externally, instead of the single great " gill-slit," cov- 

 ered by a " gill-cover," as seen in the ordinary bony-fishes, the 

 side of the neck presents seven round holes placed far back 

 in a line on each side. These holes are the external apertures 

 of the gills (Fig. 106, A), which in these fishes are in the 

 form of sacs or pouches, the lining membrane of which is 

 thrown into numerous folds or plaits, over which the branchial 



