ZOOLOGICAL SOCIETY BULLETIN. 



745 



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 the}' do in the sharks. They 

 are, however, rotated in the sockets so that they 

 look upwards instead of sidewise. The sjiiracles, 

 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 tlie head 

 behind tlie 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 embrj-o is at first elongate like that of the 

 shark, and the gill openings are on the side of 

 tlie head, but as development proceeds the body 

 becomes fl.ittened 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-fislies (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 

 liottom; second, their nearest living relatives, the 

 Zciilae, still swim in the usual fashion, and, 

 third, their probable fossil ancestors (Amphis- 

 liklae), were symmetrical fishes which certainly 

 had not become adapted to living on tlie bottom 

 and swimming on the sidi'. 



Some of the species haliitually 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 

 jjrocess 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 eves are both situated on the upper side. 



*The reader is referred to the article by Dr. F. B. 

 Sumner in the Novenibci', 1910, number of the Bul- 

 letin. 



