118 LUNG-FISHES 
is guarded by an operculum, OP. The stunted external 
gills which here protude, £2, 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, S7/V; 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, VCH, 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- a 
garded as neural, JV, VS, or hemal processes and spines, — 
the distal elements as equivalent to the basal and radial fin 
supports, B+ R. <A stout, longitudinal ligament, ZZ, 
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 air-bladder. ] 
The structure of the paired fin is normally of the 
archipterygial form of Fig. 54. In Protopterus, however, 4 
