PELAGIC AND BENTHIC FISHES, SWIMBLADDER, ECONOMY OF DEEP-SEA LIFE 103 

 myotomes is taken up by extensive fat sinuses, which are especially capacious along the back between 

 the head and the dorsal fin. Subcutaneous fat sinuses also occur between the pelvic and anal fins, 

 between the muscles moving the dorsal and anal rays, and above and below the caudal peduncle. 

 The drawing of the fish in Text-fig. 44 B is actually of the closely related species, C. pallida, in which 

 the lack of pigment in the skin makes the sinuses easier to trace. 



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Text-fig. 44. External appearance of (a) Maurolicus muelleri; (b) Cyclothone pallida. Both, x 3. 



Reference to Text-fig. 43 will show that the brain of Maurolicus is large compared to that of 

 Cyclothone microdon. (The standard lengths of the two fishes measured 35-0 and 36-5 mm. in the 

 order just given.) This is even more true of the gill surface. On the lower part of the first gill arch of 

 Maurolicus, the filaments have an average length of 1-5 mm. and there are 5 per mm. The number of 

 lamellae per millimetre of filament is about 50. The first two corresponding figures for Cyclothone 

 microdon are 0-2 mm. and 7 per mm. Once more there is ample evidence that the species from the 

 upper mid-waters is the more highly organized and active fish. 



Apart from the greater development of the myotomes in Maurolicus, there is a striking difference in 

 the nature and composition of the muscle, which may be seen in Text-fig. 43. The drawings are of 

 cross-sections taken just in front of the caudal peduncle. Here the myotomes of Cyclothone microdon 

 are formed of one type of large muscle fibre, each with a diameter of about o-i mm. Maurolicus has 

 two types of muscle fibre. The inner parts of the myotomes consist of fibres with about half the 

 diameter of those of Cyclothone, while those forming the outer parts are about half as small again. 

 These latter are presumably red fibres. 1 As the cross-sectional area of the inner parts alone is about 

 equal to the corresponding area of myotomes in C. microdon, it will be seen how much finer in texture 

 are the muscles of Maurolicus. Study of the density of the fibrils in the muscle fibres of these two 

 species would be of interest. 



1 Red muscle also forms a considerable part of the myotomes of the tunny-like fishes (Kishinouye, 1923), which are the 

 most active of all teleosts. 



