550 LOWEST VERTEBRATES. 



other known animals. Consequently we may classify the animal kingdom as 

 follows : — 



I. Chord ate Animals — Division Chordata -! Ner ™ u s system Domi ; a 



1 Notochord, and Gill-Slits. 



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-Chordate Animals — Division lNVERTEBRATA| NervousSystem Vent , ral;no 



*• Notochord or Gill-Slits. 



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 chimseroid 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 



