440 HENRY C. TRACY 



the subject have their source in his thorough and accurate 

 description. 



Weber shows that the ear-swimbladder relation is structurally 

 very different in different cases; in the five groups of fishes in 

 which he found this relation, he described three entirely dif- 

 ferent anatomical types. In later times, the list of species has 

 been extended in which the swimbladder is known to be in rela- 

 tion to the membranous labyrinth; yet no one has been able to 

 add any distinctly new types to those described by Weber. 

 These three types may be briefly summarized as follows: 

 • 1. The primitive type. This type is essentially a relation of 

 simple apposition of a precoelomic diverticulum of the swim- 

 bladder to the base of the skull. In many cases there is a 

 membrane-covered foramen in the cranial bones between the 

 swimbladder diverticulum and the membranous labyrinth. There 

 are many varieties of this type. It is found in widely separated 

 groups of fishes, e.g., Megalops, Notopterus, Sparidae, and 

 species of Serranidae and Gadidae. Probably it occurs in other 

 species not yet investigated. 



2. The clupeoid type. In this type a diverticulum of the 

 swimbladder in the form of a minute capillary tube extends into 

 the head on each side, enters the skull, and ends in two large 

 expanded vesicles which occupy an extensive and complicated 

 cavity in the bones of the lateral and basilar region of the skull. 

 The anterior vesicle comes into a definite relation with the 

 utriculus. This arrangement, so far as is know^n, is found only 

 in the Clupeoids and nearly related families. 



3. The Weberian mechanism. In this type there is an articu- 

 lated chain of small bones developed from certain anterior 

 vertebrae, which serve to connect the anterior end of the swim- 

 bladder with the perilymph cavity. The two sacculi of the 

 opposite sides are connected by a duct to which is attached a 

 medium unpaired sac. This mechanism is found in the families 

 Cyprinidae, Siluridae, Characinidae, and Gymnonoti. The ex- 

 istence of this specialized apparatus in these families suggests a 

 common descent and has led to the formation of the order 

 Ostariophysi. 



