572 



PISCES FISHES. 



skull. The tail is sometimes quite symmetrical or 

 diphycercal, but in most cases it is heterocercal at first, 

 and acquires a secondary symmetry termed homocercal ; 

 for while the end of the notochord in the young forms is 

 bent upwards as usual, the subsequent development of rays 

 produces an apparent symmetry. The scales are in most 

 cases relatively soft. The roof of the fore-brain is without 

 nervous matter. The optic nerves are remarkable, because 

 they cross one another without interlacing (decussate). 

 The partitions between the gill-clefts disappear ; so, instead 

 of the pouches seen in Elasmobranchs, there is, on each 

 side, one branchial chamber, covered over by an opercular 

 fold. Into this chamber the comb-like gills, borne by 

 the branchial arches, project freely. There is usually a 



FIG. 306. The goldfish (Cyprinus auratus). 



rudimentary gill or pseudobranch associated with the hyoid. 

 There is no spiracle. In most, a swim-bladder is developed 

 from the dorsal side of the gullet. The duct of the swim- 

 bladder may remain open (Physostomous), as in herring, 

 salmon, and carp ; or it may be closed (Physoclystous), as in 

 perch and cod. There is no spiral valve in the intestine, 

 and the food canal ends in front of, and separate from, the 

 genital and urinary apertures or aperture. The base of 

 the ventral aorta is swollen into -a non-contractile bulbus 

 arteriosus, but there is no conus, unless very exceptionally, 

 as in Butirinus. A remarkable peculiarity is that the 

 gonads are usually continuous with their ducts. The ova 

 are numerous, usually small and fertilised in the water. 

 The segmentation is meroblastic, and there is usually a 

 distinct larval stage. 



