.^^ THE DOGFISH. PHYLUM CHORDATA 



balance, but there is no evidence that the dogfish can hear. 

 Related to the semicircular canals is the system of lateral Hne 

 or neuromast organs, supphed by branches of the seventh, 

 ninth and tenth nerves. The sense cells of the system are situated 

 in pits (Fig. 246 n.aw.) and tubes in the skin filled with mucus, 

 and opening to the exterior at intervals. The most conspicuous 

 of them is the lateral line itself, which is visible as a streak of 

 pigment all the way down the side of the body. The sense cells 

 respond to any movements in the water which affect the fluid 

 in the tubes. The lateral Hne system is of great importance in 

 fishes, but is absent from terrestrial vertebrates. The ear appears 

 to be derived from the lateral line system, and the two together 

 are called the acustico-lateralis system. 



DUCTLESS GLANDS 



Finally, we may note the condition of the ductless glands 

 (p. 18) in the dogfish. With the thyroid and the pituitary body 

 we have already dealt (pp. 327, 336). The thymus is present as 

 lobed masses of glandular tissue above the gill clefts, from whose 

 epithelium it arises during development, as in all vertebrates. 

 The adrenal bodies are represented by two separate elements. 

 Between the kidneys an elongate structure, the interrenal body, 

 derived from the coelomic epithelium, represents the cortex 

 of the adrenals of the frog and higher vertebrates ; on the course 

 of the sympathetic chains a number of bodies of the same origin 

 as the cells of the sympathetic ganglia represent the medulla 

 of the adrenals. These bodies are the suprarenals. 



