150 PISCES. 



Vertebrata. From the palaeozoic formations to the chalk we find 

 almost exclusively cartilaginous fishes and Ganoids, amongst which 

 the forms with persistent notochord and cartilaginous skull pre- 

 dominate. Ganoids, with a fully-developed bony skeleton, round 

 scales and an externally homocercal caudal fin, appear for the first 

 time in the Jura, where we also find the first Teleosteans. From 

 the chalk onwards, in the more recent formations, the Teleosteans 

 increase in number and variety of forms the nearer we approach to 

 the fauna of the present time. 



Order 1. LEPTOCARDII * (ACRANIA). 



Lanceolate Fishes without paired fins. The notochord is persistent ; 

 there is no skull-capsule. The blood is colourless, and there are 

 pulsating vascular trunks. 



The body of Amphioxus (which was taken by Pallas for a slug) is 

 about two inches long. It is shaped like a lancet, and is provided 

 with dorsal and anal fin-like folds, which, however, are without rays, 

 and are continued into the lancet-shaped caudal fin. In the place 

 of the vertebral column the strong notochord persists ; on the dorsal 

 side of this is the spinal cord, the slightly swollen anterior extremity 

 of which represents the rudiment of the brain. There is no capsule 

 corresponding to the skull. There is a rudimentary eye, consisting of 

 an unpaired pigment spot, situated at the anterior end of the central 

 nervous system in the nervous tissue; also a small olfactory pit 

 placed on the left side. There is no auditory organ. 



The mouth, which is without jaws, is a long slit supported by a 

 jointed horse-shoe-shaped cartilage, bearing ciliated cirri. It leads 

 into a long and spacious sac (pharynx), which is pierced by a number of 

 lateral slits, and serves the function of respiration. At the entrance 

 of the pharynx there are two folds, and on either side three finger- 

 shaped ciliated projections. The walls on each side are supported by 



* Joh. Muller, " Ueber den Bau und die Lebenserscheinungen des Bran- 

 ch'.jstoma lubricum (Amphioxus lanceolatus)." Abhandl. der Berliner Akad., 

 1842. 



Kowalevski, " EntwickelungsgescMchte von Amphioxus lanceolatus." St. 

 Petersburg, 1867. 



Kowalevski, " Weitere Studien, etc." Arch, fur miltr. Anatomic, Tom. XIII. 



W. Eolph, " Untersuchungen iiber den Bau des Amphioxus lanceolatus." 

 Morpli. Jahr., Tom. II., 1876. 



P. Langerhans, " Zur Anatomic des Amphioxus lanceolatus." Arch, fur 

 mihrosk. Anatomie, Tom. XII. 



B. Hatschek, " Studien iiber die Entwickelung des Amphioxus." Arbeiten 

 aus dem Zool. Institute in Wicn, Tom. IV., 1881. 



