Families of Physostomi continued--_Salmonidæ_: Salmo, difficulty of distinguishing species, 630; constant specific characters, 635--hybrids, 638--sexual development, 638--migratory species and their retention in freshwater, 639--Growth of Salmonoids, 641--their domestication and acclimatisation, 641--species enumerated, 642--Smelt and Capelin, 646--Coregonus, 647--Grayling, 649--marine genera, 650. Families of Physostomi continued: Percopsidæ, 651--Haplochitonidæ, 651--Gonorhynchidæ, 652--Hyodontidæ (Moon-eye), 653--Pantodontidæ, 653--Osteoglossidæ, 653--Clupeidæ (Herrings, Anchovies, Shads, Mossbanker, Menhaden, etc.), 655--Bathythrissidæ, 663--Chirocentridæ, 663--Alepocephalidæ, 664--Notopteridæ, 664--Halosauridæ, 665--Hoplopleuridæ, 665--Gymnotidæ (Electric Eel), 666--Symbranchidæ, 668--Murænidæ (Eels, Congers, Murænas, etc.), 669. FIFTH ORDER--LOPHOBRANCHII 678 Families: Solenostomidæ, 678--Syngnathidæ (Pipe-fishes, Sea-horses), 679. SIXTH ORDER--PLECTOGNATHI 683 Families: Sclerodermi (File-fishes, Coffer-fishes), 684--Gymnodontes (Globe-fishes, Sun-fish), 686. THIRD SUB-CLASS--CYCLOSTOMATA. Families: Petromyzontidæ (Lampreys), 691--Myxinidæ, 694. FOURTH SUB-CLASS--LEPTOCARDII. Family: Cirrhostomi (Lancelets), 696. APPENDIX. DIRECTIONS FOR COLLECTING AND PRESERVING FISHES 697 ALPHABETICAL INDEX 707 INTRODUCTORY REMARKS. According to the views generally adopted at present, all those Vertebrate animals are referred to the Class of Fishes, which living in water, breathe air dissolved in water by means of gills or branchiæ; whose heart consists of a single ventricle and single atrium; whose limbs, if present, are modified into fins, supplemented by unpaired, median fins; and whose skin is either naked, or covered with scales or osseous plates or bucklers. With few exceptions fishes are oviparous. However, there are not a few members of this Class which show a modification of one or more of these characteristics, as we shall see hereafter, and which, nevertheless, cannot be separated from it. The distinction between the Class of Fishes and that of Batrachians is very slight indeed. The branch of Zoology which treats of the internal and external structure of fishes, their mode of life, and their distribution in space and time, is termed Ichthyology.[1] CHAPTER I. HISTORY AND LITERATURE. [Sidenote: Aristotle.]