All observations directed to sex, mode of propagation, and development, will have special interest: thus those relating to secondary sexual characters, hermaphroditism, numeric proportion of the sexes, time of spawning and migration, mode of spawning, construction of nests, care of progeny, change of form during growth, etc.

If the collector is unable to preserve the largest individuals of a species that may come under his observation he should note at least their measurements. There are but few species of fishes of which the limit of growth is known.

The history of Parasitic Fishes is almost unknown, and any observations with regard to their relation to their host as well as to their early life will prove to be valuable; nothing is known of the propagation of fishes even so common as Echeneis and Fierasfer, much less of the parasitic Freshwater Siluroids.

The temperature of the blood of the larger freshwater and marine species should be exactly measured.

Many pelagic and deep-sea fishes are provided with peculiar small round organs of a mother-of-pearl colour, distributed in series along the side of the body, especially along the abdomen. Some zoologists consider these organs as accessory eyes, others (and it appears to us with better reason) as luminous organs. They deserve an accurate microscopic examination made on fresh specimens; and their function should be ascertained from observation of the living fishes, especially also with regard to the question, whether or not the luminosity (if such be their function) is subject to the will of the fish.

Fig. 321.—Scopelus boops, a pelagic fish, with luminous organs.

INDEX.