112 THE VERTEBRATE SKELETON. 



ENDOSKELETON. 



SPINAL COLUMN l . 



The spinal column of fishes is divisible into only two 

 regions, a caudal region in which the haemal arches or ribs 

 meet one another ventrally, and a precaudal region in which 

 they do not meet. 



The various modifications of the spinal column in fishes 

 an be best understood by comparing them with the arrange- 

 ment in the simplest type known, namely Amphioxus. In 

 Amphioxus the notochord is immediately surrounded by a 

 structureless cuticular layer, the chordal sheath. Outside this 

 is the skeletogenous layer, which in addition to surrounding 

 the notochord and chordal sheath embraces the nerve cord 

 dorsally, and laterally sends out septa forming the myomeres. 



The CARTILAGINOUS GANOIDS 2 Acipenser, Polyodon and 

 Scaphirhynchus are the simplest fishes as regards their spinal 

 column. The notochord remains permanently unconstricted 

 and is enclosed in a chordal sheath, external to which is the 

 skeletogenous layer. In this layer the development of carti- 

 laginous elements has taken place. In connection with each 

 neuromere, or segment as determined by the points of exit of 

 the spinal nerves, there are developed two pairs of ventral 

 cartilages, the ventral arches (basiventralia) and intercalary 

 pieces (interventralia) ; and at least two pairs of dorsal pieces, 

 the neural arches (basidorsalia) and intercalary pieces (inter- 

 dorsalia). The lateral parts of the. skeletogenous layer do not 

 become converted into cartilage, so there are no traces of 

 vertebral centra. The ventral or haemal arches meet one 

 another ventrally and send out processes to protect the ven- 

 tral vessels. The neural arches do not meet, but are united 

 by a longitudinal elastic band. 



In Cartilaginous ganoids the only indications of metameric 



1 See H. Gadow and E. C. Abbott, Phil. Trans, vol. 186 (1895) B. 

 pp. 163221. 



2 C. Hasse, Zeitschr. wiss. Zool. LVII. 1893, p. 76. 



