238 VERTEBRATE ANIMALS. 
ciliated pit, which is believed to be a rudimentary organ of 
smell. 
ORDER II, MARSIPOBRANCHII (Gr. marsifos, a pouch ; drag- 
chia, gills).—This order includes the Hag-fishes (7yxinid@) and 
the Lampreys (Petvomyzonide), and-it is defined by the follow- 
ing 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 dudbus arteriosus. The gills are pouch- 
like, communicating with the throat on the one hand, and open- 
ing externally on the other by means of apertures placed on the 
sides of the neck. 
The Hag-fish (Zyxzne) is an eel-like fish, which agrees with 
the Lampreys in having neither pectoral nor ventral fins, the re- 
presentatives 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 embedded 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 disc 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-fishes (Lefzdostren), 
the nasal chambers are closed behind, and do not communicate 
with the cavity of the mouth, as they do in the higher Verte- 
brates. In the Myxinoids, however, such a communication does 
exist. The nasal sacs are placed in communication 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 Lampreys, 
is to be found in the structure of the respiratory organs, from 
which the name of the order is derived. When viewed exter- 
nally, instead of the single great “ gill-slit,” covered 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. 128, 
A), which in these fishes are in the form of sacs or pouches, the 
