356 KAMAKICHI KISHTKOCYE: 



posterior end of the j)rincipal cavity. At the anterior end of this upper 

 accessory ca'S'ity a vein poui-s to a segmentaiy vein. In immature tunnies 

 the air-bladder is very small, and almost collai^sed. The ah-bladder of this 

 species has a pair of shght sweUings along the anterior side. 



In Paratliumvus mebacJii the air-bladder is a Httle naiTower than the roof 

 of the abdominal ca\ity ; but occupies the entire length of the cavity, at the 

 anterior end the aii'-bladder is divided into two lai'ge coeca, by the dorsal 

 aorta in the middle, and is sepai'ated by the cutaneous aiieries from the princi]^)al 

 cavity. The intei'nal wall is finely reticulated. 



In Neothunnus macropterus the aii'-bladder is naiTow, and is protected by 

 a veiy thick mass of connective tissue from the venti'al side. This thick niixss 

 of connective tissue is utilized as a material in making glue. On the middle 

 of the dorsal wall a large vein is found with radiating venules fi'om all sides. 



The red gland is developed at the anteiior pait of the air-bladder, near the 

 point where the arteiy for the air-bladder entei's. The air-liladder of the Scom- 

 bridae and Cybiidae receives blood fi'om the dorsal aoiia at several points, and 

 ix)urs its venous blood to the posterior cai'dinal vein at several s^xits ; but in 

 the Plecostei the arterial blood is received from a special branch of an artery, 

 rimning along the right hand side of the stomach, and the venous blood poui-s 

 to the caudal or the posterior cardinal vein through a segmental vein. Thus 

 the aaierial system of the aii-bladder belongs to the axial system in the Scom- 

 biidae and Cybiidae, but to the visceral system in the Plecostei. 



DIGESTB^E SYSTEM. 



The mouth-cavity is black in the Scombridae, black or gi-c;s'ish in the 

 Cybiidae and Thunnidae, and silvery or colomlcss in the Katsuwonidae. The 

 tongue is small, naiTow, and black in colour, and fai' behind the symphysis of 

 the lower jaw in the Scombridae ; broad, flat, and genei-ally gi'eyish in the Cybi- 

 idae ; greyish in the Thunnidae ; and silvery white, medium in size, and the 

 membrane at the lateral mai'gins is tm-ned upwai'd in the Katsu'nonidae. The 

 surface of the tongue is gi-auulated in the genus Scomber, armed with villous 

 teeth in Gymnoaarda and Thunnidae, and quite smooth in Acanthocybium, in 

 many species of the genus Oybium, and in the fishes of ithe Katsuwonidae. 



The development of tlio gill-rakei-s on the branchial arches , has a dose 



