548 ORGANS OF RESPIRATION. 



branchiate species, the syngnathus, peyasus, and hippocampus 

 present a regular series of ciliated filamentous tufts, disposed 

 along all the branchial arches, and much resembling the 

 ramified branchial tufts of many gasteropods and annelides ; 

 these filaments are free at their ends, concealed in a branchial 

 cavity, and communicate externally by a single operculated 

 opening on each side, as the gills of other osseous fishes. 

 In the heterobranchus anguillaris, there are also small acces- 

 sory ramified branchial tufts, in addition to the usual 

 laminated gills. 



Besides the ordinary internal, covered, laminated branchiae, 

 many cartilaginous fishes, as the rays and sharks, and some 

 osseous fishes, present, in the foetal state, long, external, 

 simple, branchial filaments, continuous with the internal 

 laminae of the permanent gills, and hanging down free from 

 the sides of the exterior branchial openings. These deci- 

 duous, exterior, branchial filaments are lost in the foetus of 

 the rays and sharks, before they escape from the ovum ; and 

 in the osseous fishes, where they have been observed, they 

 disappear by absorption at a very early period of foetal life. 

 In watching the development of the permanent branchial 

 apparatus of fishes, the outer surface of the neck appears, 

 at an early period, to have no branchial openings ; but there 

 are soon after formed five small separate holes on each side, 

 even in the operculated osseous fishes. The branchial 

 arches at length make their appearance, and the minute 

 constituent laminae of the gills begin to bud out from their 

 edges. The laminae increase in number, in extension, and in 

 firmness of texture, and hang free from the sides of the neck ; 

 the opercular apparatus now begins to be developed, and by 

 the gradual extension of its branchiostegous membrane and 

 rays, the branchial organs are covered and concealed. 



Most fishes have an air-sac in the abdomen, extended 

 longitudinally under the vertebral column, exterior to the 

 peritoneum, and containing a gaseous secretion, differing in 

 its constitution from atmospheric air. The air- sac is most 

 developed in species which frequent, or feed at the surface of 

 the water, and is least developed or wanting in those which 

 lie at the bottom or burrow in mud ; its secretion contains 

 a larger proportion of oxygen in the powerful predaceous 

 fishes of deep seas, and nitrogen predominates in the feebler 



