DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 919 



is placed. Sensory hairs appear to occur on the surface of this cushion. Similar sensory 

 organs also occur in a tube hollowed out in the mandibular cartilage. These sensory 

 organs communicate with the exterior, since the tube in which each is placed is in its 

 posterior part incomplete below, and the columnar epithelium on the roof of the cavity is 

 directly exposed to the surrounding medium and to stimuli from the outside. 



Alimentary System. — The mouth in the larval Anarrhichas presents a different 

 shape from that of the salmon, and on emergence from the egg it is moreover widely 

 open, forming a lozenge-shaped aperture when viewed from the front, and its border is 

 rigid and motionless. Its upper angle (the premaxillary elements) is considerably elevated 

 dorsally, and is depressed between the very large eyes. In this form and in the salmon a 

 peculiar flapping of a process is observed in respiration, and in sections of both species an 

 extension of the mucous membrane of the mouth hangs down and projects inward from 

 the jaws on each side of the trabecule (PL XXIII. fig. 3, Jim). This membrane forms a 

 complete floor in front, but posteriorly it runs into lateral flaps, which become continuous 

 with the mandibular articular process. In Anarrhichas the smooth fold of this flap poste- 

 riorly, as its free tip bends in to join the upper surface, indicates a definite differentiation. 

 In the young salmon the membranous process waves synchronously with the movement 

 of the mandible. The anterior margin of the mouth, in transverse section, forms at first 

 a well-marked groove with the lumen of a vessel in each mandibular elevation. It is to the 

 latter that the soft tissues of the mandible are soon attached. The mucous lining of the 

 roof of the mouth, from the point of junction of the mandible, is comparatively thin, 

 the cells being chiefly of the tesselated variety. 



In Cottus, § inch long, the oral mucous membrane shows large glandular cells, spher- 

 ical in form, and disposed along the numerous rugse which are better developed in 

 Callionymus. In this last-named species, when ^ inch in length, the mucous layer is very 

 thick and the rugse most marked. A great increase of glandular epithelium may occur in 

 the lining of the branchial region, and in Labrus, T 7 ^ inch long, it forms a layer of some 

 thickness at the upper angle of the opercular flap. The mucous coat becomes thicker, 

 and shows columnar cells in Anarrhichas, when the notochord appears in section, and 

 the submucous connective tissue increases in volume, so that the internal lining is readily 

 thrown into frills. Outside the sub-mucous coat is a layer of circular muscular fibres. 

 The frills of the mucous membrane are rendered more prominent by the grasp of the 

 increasing circular coat, and the canal is diminished in calibre, so that it forms a com- 

 paratively small tube (PL XXVII. fig. 6). The columnar and finely granular epithelial 

 lining also has considerably increased in thickness. 



In the post-larval Gadoid, § inch in length, the general structure of the oesophageal 

 wall is similar, but the grooves in the roof of the pharynx are deep — two ridges projecting 

 very prominently — while on the floor three or more ridges occur. The epithelial coat 

 passes into the grooves, and is very largely developed — the bulky mucous cells resting 

 upon a loose meshwork of connective tissue, with apparently some longitudinal muscular 

 elements. A little further back glands become more abundant, and an outer tunic of 



