58 



are captured in greater numbers than females, but fishermen are unable to distinguish 

 the males from unripe or immature females. 



One part of the viscera, which has the same relations in both sexes, still remains 

 undescribed, namely, the anterior part of the intestine, and the gills which are con- 

 nected with it. The anterior part of the intestine itself needs no very elaborate 

 description : it consists simply of the mouth and throat, the walls of which differ only 

 from the stomach in the fact that they are almost everywhere continuous with the 

 surroiinding muscles and other tissues, instead of being separated from them by part 

 of the body cavity. Beneath the throat there is a part of the body cavity entirely 

 separated from the rest, namely, the pericardium, which contains the heart, and which 

 will be described below. The teeth previoush^ mentioned as connected with or 

 embedded in the superior and inferior pharyngeal bones of course project through the 

 walls of the throat, and help to masticate or grasp the food. The series of basihyal 

 and basibranchial bones, clothed with mucous membrane, form a pointed conical tongue 

 in the floor of the mouth, which is used in swallowing. 



At the sides of the throat is the important respiratory apparatus. Between the 

 branchial arches are a number of clefts by which the cavity of the throat opens to 

 the exterior. The bony arch is clothed with connective tissue, muscle, and mucous 

 membrane, and this mucous membrane becomes continuous through the cleft with the 

 outermost layer of the skin on the external surface of the body. The most anterior 

 cleft is between the hyomandibular bone and the first branchial arch : the cleft extends 

 upwards to the middle of the epibranchial bone, downwards to the end of the cerato- 

 branchial. The 2nd, 3rd, and 4 th clefts become gradually shorter, scarcely extending 

 upwards beyond the cerato-branchials. The 5th cleft is very short, it is situated 

 between the 4th cerato -branchial bone and the 5 th cerato-branchial or lower pharyn- 

 geal bone. The latter is continuous^ connected by muscle and connective tissue \ritli 

 the pectoral arch ; there is no cleft behind it. On the outer face of each arch there is 

 a double series of long straight filaments, the branchial filaments. Each of these is 

 supported by a rod of fibrous skeletal tissue which runs up its centre, and between 

 this rod and the epithelium, or layer of delicate cells which covers the surface of the 

 filament, there is a rich plexus of capillary blood vessels. The blood coursing through 

 these gives out its carbonic acid to the sea water which passes through the gill clefts 

 and absorbs oxygen from the sea water, which oxygen is necessary for the oxidization 

 of the tissues that goes on throughout life. 



In front of the first branchial cleft, running in an antero-posterior direction. beneath 

 the dorsal part of the hyomandibular bone, is a rudimentary branchia, composed not 

 of Ion" branchial filaments, but a few vascular folds of the mucous membrane : this 

 is called the pseudo-bran chia and is found in the majority of bony fishes. The pseudo- 

 branchia represents part of the branchia of the hyoid arch. On the inner surface of 

 the branchial arches there are small fleshy projections ; in many fish these are long 

 projecting rods called the gill rakers. In the sole they are rudimentary. 



