132 



PISCES. 



In the Myxinoids and Acipenseridct; these organs have the form of 

 short sacs- in the Rays, Skates, and Chimaeras they are simple tubes, 

 which begin as ampullae and extend also over the head in several 

 rows. In the Teleostei there are branching tubes which pierce the 

 scales of the lateral lines as pores, and are also present on the head in 

 several rows (fig. 583). Nerves run in the walls of these tubes and 

 end in knob-like swellings. The epithelial covering of the latter 

 contains in the centre short piriform cells, which at the free end are 

 prolonged into a fine stiff hair, while at the base they pass into a 

 varicose process the axis cylinder of a nerve fibre (fig. 584). 



The skeleton in its simplest form consists only of the notochord 

 (Amphioxus). The notochord also persists in the Myxinoids, which 



possess a cartilagino-mern- 

 branous cranial capsule. 

 In the Petromyzontida* 

 there appear for the first 

 time, above the notochord, 

 cartilaginous neural arches, 

 and similarly beneath it 

 paired cartilaginous bands. 

 These are the first rudi- 

 ments of the dorsal and 

 ventral vertebral arches. 

 These vertebral arches are 

 more perfect in sturgeons 

 (Acipenser), and in the 



FIG. 584. a, Lateral organ in the tail of a fish (roach); /7 



N, nerve, b, lateral organ in the head of a young sea - cats (ChimCBrct), 111 

 fish (bream) (after P. B. Schulze). whi(}h the noto chord per- 



sists, surrounded by a very compact connective-tissue sheath. A 

 differentiation of the axial skeleton into separate vertebrae is first 

 found in the Skates and Rays, where dorsal and ventral arches 

 are united with annular portions of the notochordal sheath which 

 become cartilaginous vertebral bodies. The notochord is constricted 

 by the growth of the latter in the centre of each vertebra, in such a 

 manner that biconcave (cmvphiccdous) vertebral bodies are formed, 



* Compare Job. Muller I.e., Reichert, " Ueber die Visceralbb'gen im 

 Allgemeiuen, etc." Mailer's Archir, 1837. 



A. KSlliker, " Ueber die Beziehungen der Chorda dorsalis zur Bildung der 

 Wirbel der Selachier und einiger anderer Fische." Wiirzburg, 1866. 



C. Gegenbaur, " Ueber die Entwickelung der Wirbel saiile des Lepidostens mit 

 vergleichenden anatomischen Bemerkungen.'' Jot. natwrcissensch. Zeitschr., 

 Tom. III. 



