. Elementary text-book of zoology. Zoology. 144 PISCES. which swim very well are without the swimming bladder is by no means favourable to the interpretation of its function. When it is present the fish must have the power of compressing it, partly by the muscles in its walls and partly by the muscles of the body, and thus rendering the body specifically heavier so that it sinks. When the compression of the muscles is removed the compressed air will again expand, the specific gravity diminish and the fish will rise. If the pressure is unequal on the anterior and posterior parts then that half
. Elementary text-book of zoology. Zoology. 144 PISCES. which swim very well are without the swimming bladder is by no means favourable to the interpretation of its function. When it is present the fish must have the power of compressing it, partly by the muscles in its walls and partly by the muscles of the body, and thus rendering the body specifically heavier so that it sinks. When the compression of the muscles is removed the compressed air will again expand, the specific gravity diminish and the fish will rise. If the pressure is unequal on the anterior and posterior parts then that half of the fish which is rendered specifically heavier will sink. Still more complicated relations, however, seem to exist according to the investigations of Bergmann.* Respiration is in all cases effected by gills. In the Cyclostomes (fig. 592) which have no visceral arches there are six or seven pairs of branchial pouches. These open into the oeso- phagus either by internal branchial passages or (Petromyzon) by a com- mon canal which receives all the branchial passages. The water is expelled through external branchial passages round which a network of cartilaginous rods is developed. In the Plagiostomes (fig. 593 a) there are saccular spaces the walls FIG. 59*.—Head of Anabas seandens of which are supported by car- (regne animal). The operculum . i rri l «V,inl has been removed to shew the tilaginous rods. These branchial spacious upper pharyngeal bones sacs communicate with the exterior (pharyngo-branchials). , , . , by lateral openings and contain the branchial leaflets which are attached to their walls: they are separated from one another by partition walls which are placed between the two rows of leaflets of each arch, and they are supported by an external framework of cartilaginous rods. In the Selachians there are, as a rule, five pairs of branchial sacs, of which the last has a row of leaflets on its anterior wall only, , on the posterior side of the fourth true bra
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
