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 
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a 
Fic. 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. 
