156 FISHES CHAP. 
free opercular margin fuses below with the isthmus, or behind 
with the side of the head. Thus the aperture may extend from 
the chin in front upward and backward to near the dorsal sur- 
face of the head, or it may be reduced to little more than a mere 
pore situated on any part of the opercular edge (e.g. Hippocampus); 
or, as in Symbranchus, the reduced pores of opposite sides may 
coalesce in the floor of the throat in a common median opening. 
In the Elasmobranchs and in the Dipnoi the cloacal aperture 
is always situated at the junction of the trunk with the tail. In 
the Teleostomi, however, where the intestine has a separate ex- 
ternal orifice or anus, distinct from, and placed in front of, the 
separate or combined urino-genital ducts, the anus may either 
retain its primitive position near the union of the trunk and 
tail, or occupy almost any intermediate position between this 
point and the throat. 
Most Fishes possess both median and paired fins (Fig, 93, A). 
From an evolutionary point of view the median fins have a far 
greater antiquity than the paired fins. They appear before the 
latter in embryonic development, and in the Cephalochordata, and 
such lower Craniates as the Cyclostomata, they are the only fins 
which exist. The isolated median fins of most Fishes are discon- 
tinuous remnants of a primitively continuous structure, which, 
extending like a fringe along the median line of the back, was 
thence continued round the end of the tail and forward along 
the ventral surface as far as the cloacal or anal orifice. This 
primitive condition, which, as we have seen, is characteristic of 
Amphioxus, is also very general in the embryos and larvae of 
Fishes (Figs. 258 and 309), and is more or less completely 
retained in the Dipnoi and in many adult Teleosts, notably in 
those species in which the body is greatly elongated and locomo- 
tion is effected by serpentine lateral undulations, as in the Eels 
(Anguillidae), and in others which, either through their quasi- 
parasitic or commensal habit (e.g. Fierasfer acus), or by reason of 
a peculiar environment, as in certain deep-sea Fishes (Fig. 430) 
are distinguished by the retention of many larval features. More 
generally, however, the continuity of the fin becomes inter- 
rupted, and that portion of it which surrounds the extremity 
of the tail is the first to become separated from the rest as 
a caudal fin (Fig. 429). By further interruptions the remaining 
dorsal portion may become divided into two or three isolated 
ne meted 
