ZOOLOGICAL SOCIETY BULLETIN. TA5 
COMMON SKATE. 
Under side. 
under the head, a position quite unusual in fishes. 
The gill openings, which in the sharks are later- 
ally situated, are here placed in two divergent 
rows on the ventral side. 
The upper side also shows some unusual fea- 
tures. The eyes are on top of the head, but 
they occupy about the same position relative to 
the brain case that they do in the sharks. They 
are, however, rotated in the sockets so that they 
look upwards instead of sidewise. The spiracles, 
which are vestigial gill slits and are either small 
or wanting in most sharks, are large in the 
skates and are situated on the top of the head 
behind the eyes. They have the unusual func- 
tion of admitting the water to the gills, an act 
ordinarily performed by the mouth. They are 
supplied with valves which open and close 
rhythmically. Professor Rand has shown that 
a strong current may be ejected through the 
spiracles, apparently for the purpose of cleans- 
ing the gills. 
Our common skates lay large eggs encased in 
horny shells, but many species are viviparous. 
The embryo is at first elongate like that of the 
shark, and the gill openings are on the side of 
the head, but as development proceeds the body 
becomes flattened and the gill openings move into 
a ventral position. This of course repeats what 
has happened in the evolution of the group. 
Even in the adults there are various degrees of 
adaptation, and some species, e. g., the saw-fish, 
show very plainly the relationship to the sharks. 
The flounders or flat-fishes (Pleuronectidae ) 
are among the most highly specialized of the 
bony fishes. As already indicated they once 
swam in a vertical position like other fishes, but 
on assuming a bottom habitat they became adapt- 
ed to lying on one side and to swimming in this 
position. It is easy to understand how this may 
have come about, for many fishes which swim in 
the ordinary position often rest on one side on 
the bottom. A visit to the Aquarium will dem- 
onstrate this fact to anyone who can catch the 
trigger-fishes, the tautog and certain other spe- 
cies in a siesta. 
We have positive evidence of three different 
categories that this change in the position of the 
body has really taken place. In the first place, 
the newly hatched young of the flounder swim 
in the position normal to other fishes and turn 
on the side only when they begin to live on the 
bottom; second, their nearest living relatives, the 
Zeidae, still swim in the usual fashion, and, 
third, their probable fossil ancestors (Amphis- 
tiidae), were symmetrical fishes which certainly 
had not become adapted to living on the bottom 
and swimming on the side. 
Some of the species habitually turn on the 
right side and others on the left, while in still 
others either the right or left may become the 
lower side. 
Now let us consider the changes which this 
process of adaptation to bottom life has brought 
about: the body has been but little modified, but 
the side upon which the fish habitually lies is 
more flat than the upper side, enabling it to fit 
more closely to the bottom. The lower side is 
colorless, as the pigment has been lost, while the 
upper side is well provided with pigment for 
protective coloration. It is interesting to note 
that this pigment is usually symmetrically dis- 
tributed over this side just as though it repre- 
sented right and left sides.* 
The most important structural asymmetry is 
found in the head region, for, while the gills 
are symmetrically placed on the upper and lower 
sides and the mouth opens in the normal fashion, 
the eyes are both situated on the upper side. 
" *The reader is referred to the article by Dr. F. B. 
Sumner in the November, 1910, number of the But- 
LETIN. 
