Adaptations of Larval and Post-Larval Stages to a Pelagic Life. 19 



variation in length of the first ray in young specimens and its greater development 

 in adult specimens cannot be held to prove, as is suggested by Gunther, that this 

 ray is exposed to injury and is reproduced when lost. He neither knew the course 

 of development of this ray, nor does he give the absolute lengths of the young 

 specimens examined. 



The Adaptations op Lophius Piscatoeius to a Prolonged Period of 



Pelagic Existence. 



Hitherto, the absence of complete information of the different developmental 

 forms has led to the ^new that the j^elagic stages of existence were passed through 

 .very quickly. For the North Sea, where sheets of spawn had occasionally been 

 recorded, there was no definite information as to the post-larval stages, the youngest 

 stages known being bottom forms which had reached a length of 12 cm. Fulton, 

 therefore, reasonably assumed that the absence of records of the small forms was 

 due to the fact that the pelagic life was very short, and that these forms soon 

 reached a habitat, such as the sea-weed zone, where they were inaccessible to the 

 ordinary fishing apparatus. On the other hand, the spawn and very early stages 

 were unknown to Stiasny from the Gulf of Trieste, where numerous older post- 

 larval stages were caught during three months of the year. He, therefore, also 

 concluded that the pelagic stage of LopJiius was of very short duration. 



It has already been stated that individual eggs, freed from a sheet of spawn 

 recently extruded, floated at the surface of sea-water and appeared like normal 

 large pelagic fish eggs. Pieces of the spawn with the contained eggs also floated 

 at the surface, but the gelatinous material itself when freed from the eggs sank 

 slowly. 



The evidence which may be deduced from previous observations on the sheets 

 of spawn — whether they float for a time at the surface or are fomid normally in 

 the lower water layers — is not convincing. 



The remarks made by Milroy (1897) on the egg masses of LopJiius in his 

 researches on the physical and chemical changes taking place in the ova of marine 

 teleosteans during maturation are of interest. " The ova of LojjMus piscatorius 

 may be perhaps regarded as a class distinct from the ordinary demersal ova. When 

 immature they have a volume of aboui: "268 cubic mm., when mature 4*18 to 

 5 cubic mm. These eggs are of especial importance, as they are examples of ova 

 which are dependent for their floating capabilities, not on their specific gravity, 

 but on that of the gelatinous sheet in which they are embedded. Again, the 

 specific gravity of the interstitial mucoid-like tissue, with the eggs embedded in 

 it, was I'OOS, while the ripe ova alone had a specific gravity of I'OSO to 1"035. In 

 the ripe Lophius roe 20"22 per cent, of the total weight is due to the ova, and the 

 rest to the mucoid-like tissue." It is to be observed, however, that these experi- 

 ments were made on ova which had not been spawned, and the specific gravity 

 given for the eggs is probably higher than it would be for eggs which had been 

 extruded. 



If the table of records of isolated eggs caught in the North Sea be examined, 

 it will be found that most of these were caught in the tow-nets in the surface layers. 



Observations made on larvae newly hatched from isolated eggs captured in the 

 North Sea showed that a considerable quantity of unused yolk still remained, and 

 that these larvae were able to float at the surface. It was not until the yolk had 

 been considerably reduced that they sank to the bottom of the vessel. 



According to Mr. Dunn, Mevagissey, Cornwall, who sent a sheet of spawn to 

 Cunningham, he saw the spawn from the cliffs as a dark-coloured patch in the 

 water, and next day took a boat, found it, and brought it ashore. 



Meek (1903) states that a sheet of spawn, obtained off the coast of Northumber- 

 land, was caught five fathoms under the surface, and Lo Bianco (1908-9) also records 

 the fact that a portion of spawn from the Mediterranean was found floating three 

 metres under the surface. Williamson (1911) has suggested that the sheet of 

 spawn may float some distance below the surface water, but gives no reason for 

 this view. In these three cases it is known that the embryos were well developed. 

 A large mucoid band obtained by Fulton (1902) in a trawl haul made in Aberdeen 



