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 o f which are supported by car- 



(regne animal). The operculum . i rri l V,i n l 



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, i.e., on the posterior side of the 

 fourth true branchial arch ; while the first pouch has, in addition 

 to the anterior gill of the first branchial arch, a gill on the 

 hyoid arch corresponding to the accessory gill of Chimera and the 

 Ganoidei. The mandibular arch, however, sometimes bears a 



* Compare Bergmann and Leuckart, " Anat. Phys. Uebersicht des Thier- 

 rcichs." Stuttgart, 1852. 



