RETROSPECT AND CONCLUSIONS 155 



Scabbard fisli, known as Espada, and the Rabbit fish or Prome- 

 tliit'lithys pioinotluMis. which have eyes as Large in circumference 

 as a two-shilling ])iece, but yet have optic lobes of average size. 

 At depths of 300 to 400 fathoms, approximately half a mile down, 

 there can be very little light and the large eyes are adapted to receive 

 as many photons as possible. 



If the eyes of a plaice are removed they will be found to lie in 

 a membranous covering, filled with fluid, and this communicates 

 with another sac situated ventrally which also contains fluid. 

 By means of a muscle external to the lower sac, the fluid is driven 

 into the sac lining the orbit and thus the eyeball is protruded. 

 It would appear that this accessory organ is evolved as an aid to 

 the phenomenon of protective coloration. We can daily observe 

 that in addition to the black and yellow cliromatophores which 

 produce the various markings in the plaice that there are bright 

 red markings. The eyes of this fish are also well known for their 

 marked prominence. It is a matter for thought and consideration 

 whether the bi-lobed optic lobes of the plaice may not be associated 

 with the function of protective coloration. It has been noted how 

 that in the gudgeon, the development of barbels with their taste- 

 buds has produced a division of the facial lobe into anterior and 

 posterior lobes and that this anatomical concUtion is associated with 

 the grubbing habits peculiar to the species. On the same principle 

 there may have developed a special area in the tectum opticum 

 connected with the power of self- concealment by colour changes ; 

 tliis would explain the bi-lobed character of the plaice's optic lobes, 

 and also the bulge of the lemon sole's optic lobe. 



The fact that these fishes feed by sight does not seem to explain 

 the condition, as many sight feeding fishes exist which have large 

 eyes and well developed optic lobes, but show no modifications of 

 these lobes of a similar nature. If the eyes of a sole are dissected 

 the minute size is very impressive and when the dissection is carried 

 fiu-ther it is difficult to make out any recessus orbitalis although 

 there is a slight projection usually of the cavity of the orbit. 



The more the phenomenon of protective coloration in flat- 

 fishes is considered, the more complex and mysterious is the mechan- 

 ism of its evolution. 



The various physiological and anatomical devices that have 

 been evolved, to perfect these fishes in adaptation to their sur- 

 roundings, are very complex. To the naturalist that probes 

 into the details of the modifications of structm-e, that complete 

 the picture, a feeling of wonder at the results of random variation 

 must arise. 



