428 FISHES. 



the notochord persists unsegmented ; the paired fins have a 

 central axis; the tail is symmetrical (diphy- or proto-cercal) ; 

 the fore-brain has a nervous roof; the nasal sacs open 

 anteriorly just in front of the mouth and posteriorly within 

 it ; there is a cloaca, and a spiral valve in the intestine ; 

 there are abdominal pores ; the swim-bladder acts as a single 

 or double lung, and there are gills as well ; the auricle is 

 divided by a complete partition, and the ventricle by a slight 

 one, which is continued into the large spirally twisted conus 

 arteriosus ; the Mullerifin duct is developed. 



Genera. — The genus Ceratodus is represented by two species in the 

 rivers of Queensland, and by fossil remains from Triassic and reputed 

 Permian strata. The living species frequent still rivers, feed on the 

 fallen leaves of trees and other plants, and gulp air at the surface of the 

 water. It is said that they sometimes leave the water, but as their fins 

 are weak it is not likely that they travel far. The Barramunda (C 

 forsteri) attains a length of six feet, and has salmon-coloured flesh which 

 is valued as food. Of the development of this type nothing is as yet 

 known, but the eggs are like those of the newt in size, and are laid in 

 strings surrounded by a jelly. Gerhard Krefft described C. forsteri m 

 1870, and about the same time Giinther investigated the structure of C. 

 miolepis, and recognised the importance of the type. 



Protopterus annectens lives in fresh water in tropical Africa, especially 

 to the west. It feeds on fish, frogs, insects, and other small animals. 

 The limbs are linear, and are moved like the legs of a newt. Besides 

 the paired lungs and the internal gills, there are three external gill-like 

 outgrowths, like those of young tadpoles. When the ponds dry up, the 

 fish buries itself in the mud, and remains there till the rains refill the 

 pools. Hardened "mud-nests" have been dug out and brought to - 

 Europe without injury to the enclosed Protopterus. 



Of Lepidosiren paradoxa, discovered about fifty years ago by Natterer 

 in the rivers of Brazil, little is known, but it seems to be only a species 

 of Protopterus. 



The limb of Ceratodus is trowel-shaped, and consists of a central axis 

 with bilateral rays ; that of Protopterus is linear, the central axis bearing 

 but a slight lateral membrane without rays. 



In Ceratodus the lung is single ; in Protopterus it is double. The 

 tube to the lung springs from the ventral side of the gullet. 



In Ceratodtis there are four gills on each side ; in Protopterus there 

 are two and a half. Moreover, in Protopterus there are the external 

 gills already mentioned. 



As a pulmonary circulation has been established, the heart is more 

 complex than in other fishes. It is like the heart of Amphibians. The 

 sinus venosus is divided into two sections — a right systemic, a left 

 pulmonary; the cavity of the auricle is also divided. The spirally 

 twisted conus arteriosus is divided into two channels by a complete 

 septum in Protopterus ; in Ceratodus there is a complex system of valves, 

 but the division into two paths is incomplete. 



