THE LAMPREYS AND HAGFISHES 2927 



I. CHORDATE ANIMALS Division CHORDATA 



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) L,ampreys and Hagfishes Class CYCLOSTOMATA. 



2. Semivertebrates Subkingdom PROTOCHORDATA. 



(1) Lancelets Class LEPTOCARDII. 



(2) Sea Squirts Class TUNICATA. 



(3) Worm-like Forms Class ENTEROPNEUSTA. 



II. NONCHORDATE ANIMALS Division INVERTEBRATA ^ 



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 lyAMPREYS AND HAGFISHES SUBCLASS Marsipobranchii 



As a class, the lampreys and their near allies the hagfishes, with which may 

 probably be grouped certain armored 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 rasping 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 characteristics are applicable to certain extinct forms now considered 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 representatives of a subclass 

 (Marsipobranchii), apart from the aforesaid extinct forms. In the existing mem- 

 bers of the group the skeleton is cartilaginous; the skull, as in the chimaeroid 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 subclass), 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 sur- 

 rounded 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 straight and simple; and the reproductive organs 

 discharge into the cavity of the body. The place of teeth is taken in some forms 



