330 A HISTORY OF FISHES 



and Sprat {Clupea sprattus) actually hatch out from two to four 

 days after fertilisation. As in the case of the demersal eggs, a 

 low temperature delays hatching, and it has been found that 

 the eggs of the Herring {Clupea harengus) will hatch in eight or 

 nine days in water kept at a temperature of 52° to 58° F., but 

 will take forty-seven days in water of 32° F. (i.e. just freezing). 

 As might be expected, the newly hatched larvae of these fishes 

 are very small, sometimes only three millimetres in length, and 

 are imperfectly developed. At the same time, the remainder 

 of the development is sometimes crowded into an incredibly 

 short space of time, and the three-millimetre larva hatched 

 on the fourth day may have assumed the essential features of 

 the all but mature fish before a month has elapsed, and before 

 it is much more than ten millimetres in length. 



As a general rule, the larva hatched from a pelagic ^gg is 

 transparent, with the pectoral fins developed to a greater or 

 lesser extent, and with a continuous median fin-fold along the 

 back, round the tail, and along the lower edge of the body as 

 far forward as the vent or even farther : this fold has the form of 

 a simple membrane, and is not yet supported by rays. The 

 mouth is frequently not yet formed, the blood is quite colourless, 

 and even the gill-clefts may be still wanting. In this condition 

 nourishment is provided by the remains of the yolk, but as this 

 is used up the mouth is developed and the larva begins to feed 

 on the minute organisms of various kinds found near the surface 

 of the sea. At a later stage the continuous median fin becomes 

 split up into its definitive components, and the pelvics make 

 their appearance. By degrees, the form, proportions, and 

 structure of the adult fish are assumed, and, as a rule, all the 

 essential organs, including the bony internal skeleton, are 

 developed before the fish is much more than an inch in length. 

 It may be noted here that the fins vary a good deal in their 

 development in different species, the dorsal appearing before 

 the anal in some, the anal before the dorsal in others. In the 

 members of the Herring tribe [Clupeidae) the dorsal is com- 

 pletely developed well back, and then migrates forward to its 

 final position in the middle of the length of the fish. 



Frequently special larval organs are developed, which dis- 

 appear when the more permanent organs have been acquired. 

 Such structures are for the most part concerned with feeding, 

 respiration, or locomotion, and may give the larva an appear- 

 ance so unlike that of the mature fish that it has sometimes 

 been mistaken for a distinct species. 



