460 On the Development of the Food-Fishe.<i. 



One example (P. limanda) possessed two heads, one head 

 beuig normal, while the other was much confused. The 

 bifurcation occurred in the mid-region of the trunk, and it is 

 remarkable that while the alimentary tract was bifid the noto- 

 chord was not so. An abnormal example of T. gurnardus 

 again was malformed in the cephalic region, only one eye 

 being developed and situated on the ventral side of the head. 

 The otocysts were displaced, but the trunk presented no 

 unusual features. 



Conditions of Temperature &c. 

 It is unnecessary to say that temperature has great effect 

 in accelerating or retarding developmental changes. Thus, 

 in the case of O. morrhua, the stage figured by liyder as the 

 thirtieth day was reached at St. Andrews on the twentieth or 

 twenty-first, the acceleration being due to increase of tempe- 

 rature. WheU;, however, the temperature is about 40° F., a 

 rise or fall of three or four degrees appears merely to abbre- 

 viate or lengthen development by about ten or twelve hours. 

 The series of ova and embryos dealt with in the preceding 

 pao-es were not all reared at precisely the same temperature, 

 but by a constant flow of water from the sea outside the 

 laboratory the temperature is kept as low as possible, and 

 rises very gradually as the season advances. Thus from 

 March (early in the month) to midsummer the temperature of 

 the water in the tanks rose from 34° or 35° F. to 49° and, 

 occasionally, 51° F. Of scarcely less moment than tempe- 

 rature are the other conditions, such as chemical purity of the 

 water and freedom from detritus, mud, &c. These conditions 

 are secured at St, Andrews by the proximity of the laboratory 

 to St. Andrews Bay, on the beach of which the buildings 

 stand, while the harbour passes on the north and west sides of 

 the laboratory. Before flowing into the tanks the water pumped 

 from the bay is retained in a spacious supply-tank until its 

 sediment is all deposited. This course is absolutely necessary, 

 as contact with particles of sand, mud, or mucus in the water 

 inevitably proves fatal. Newly-hatched embryos are such 

 delicate organisms that very slight contact with hard sub- 

 stances (such as contact with the side of the tank) is hurtful, 

 while the slightest pressure at once produces opacity in the 

 transparent embryo, premonitory of death. On emerging the 

 young fishes swim in reversed position, yolk upward, and for 

 some time have little power of guiding their course. By 

 rearing them in tanks of large capacity contact with the sides 

 is in a great degree obviated. 



In studying the development of the food-fishes this conclusion 



I 



