55° 
LOWEST VERTEBRATES. 
other known animals. Consequently we may classify the animal kingdom as 
follows:— 
I. Chordate Animals —Division Chordata -] NerTOUs system Dorsal; a 
1 . Vertebrates—Subkingdom Vertebrata. 
(1) Mammals—Class Mammalia. 
(2) Birds—Class Aves. 
(3) Reptiles—Class Reptilia. 
(4) Frogs and Salamanders—Class Amphibia. 
(5) Fishes—Class Pisces. 
(6) Lampreys and Hag-Fishes—Class Cyclostomata. 
2. Semivertebrates—Subkingdom Protochordata. 
(1) Lancelets—Class Leptocardii. 
(2) Sea-Squirts—Class Tunicata. 
(3) Worm-Like Forms—Class Enteropneusta. 
II. Non-Chord ate Animals —Division lNVERTEBRATA-[ NervousSystem Vent , ral; no 
We shall consider briefly the suggestions that have been made concerning the 
relationships between the semichordates and nonchordates at the close of this 
volume, and therefore proceed at once to the lampreys. Before doing so it may, 
however, be as well to mention that to rightly understand the peculiarities of all 
these matters requires a considerable amount of anatomical knowledge on the part 
of the reader; and structural features will accordingly be alluded to as simply and 
shortly as possible. 
The Lampreys and Hag-Fishes,— Subclass Marsipobranchii. 
As a class, the lampreys and their near allies the hag-fishes, with which may 
probably be grouped certain armoured extinct forms, are distinguished not only 
from the fishes, but likewise from all the vertebrates hitherto described, by the 
absence of true jaws, by the single aperture of the nostrils, as well as by the rasp¬ 
ing tongue; there being no limbs or ribs, and the notochord either persisting in its 
original form or being merely surrounded by a series of calcified rings. Probably 
many or all of these characters are applicable to certain extinct forms now con¬ 
sidered as more or less nearly allied to the lampreys, and we may accordingly 
provisionally regard these as distinctive of the subclass. On the other hand, we may 
consider the under-mentioned features distinctive of the lampreys as the represen¬ 
tatives of a subclass (Marsipobranchii), apart from the aforesaid extinct forms. In 
the existing members of the group the skeleton is cartilaginous; the skull, as in 
the chimasroid fishes and some of the sharks, is immovably joined to the vertebral 
column; and the gills are in the form of fixed pouches (hence the name of the sub¬ 
class), without gill-arches, and either six or seven in number, with their external 
apertures usually opening on the sides of the neck. Anterior in position, and 
adapted for sucking, the mouth is surrounded by a circular or subcircular lip 
supported by cartilages. The naked body is provided with median fins, having 
cartilaginous rays like those of many fishes. Internally, the heart is devoid of 
the anterior expansion known as the bulbus arteriosus; the intestinal canal is 
