Il8 LUNG-FISHES 



is guarded by an operculum, OP. The stunted external 

 gills which here protude, EB, are sometimes looked upon 

 as significant of an ancestral condition (Garman, Wieders- 

 heim). 



The viscera are somewhat shark-like in their features. 

 They include a short digestive tract, with well-marked 

 spiral intestinal valve, SIV\ a fenestrated dorsal mesen- 

 tery, DM\ a large, elongate liver, Z; a heart whose 

 arterial cone, CA, contains transverse rows of valves ; a 

 cloaca, abdominal pores (or pore), A ; and a rectal caecum, 

 CC (v. p. 263). The elongate kidney, K, the ovary, OV, 

 with its many small eggs, and the long, paired, sacculated 

 air-bladder (lung) may be named as among the least shark- 

 like of its visceral characters. 



The skeleton of a Dipnoan (Fig. 122) is almost entirely 

 cartilaginous. A stout notochord, encased in a heavy 

 sheath, NCH, passes from the skull to the tip of the tail : 

 vertebral centra encroach upon it only in the caudal region, 

 C. Dorsal and ventral processes, arranged in metameral 

 sequence, extend from the notochordal sheath outward and 

 become distally the cartilaginous supports of the dermal 

 unpaired fin. The proximal elements might thus be re- 

 garded as neural, N, NS, or haemal processes and spines, 

 the distal elements as equivalent to the basal and radial fin 

 supports, B + R. A stout, longitudinal ligament, LL, 

 serves to connect the outer ends of the cartilaginous 

 processes, as well as the proximal ends of the dermal fin 

 rays. The ribs are probably the homologues of the haemal 

 processes ; the most anterior pair, greatly enlarged, extends 

 downward on either side as the occipital ribs, OR, special- 

 ized in the function of the ai .-bladder. 



The structure of the paired fin is normally of the 

 archipterygial form of Fig. 54. In Protopterus, however, 



