The _pectoral_ fins (with their osseous supports) are the homologues of the anterior limbs of the higher Vertebrata. They are always inserted immediately behind the gill-opening; either symmetrical with a rounded posterior margin, or asymmetrical, with the upper rays longest and strongest; in Malacopterygians with a dorsal spine the upper pectoral ray is frequently developed into a similar defensive weapon. The _ventral_ fins are the homologues of the hind-limbs, and inserted on the abdominal surface, either behind the pectorals (_Pisces_ s. _Pinnæ abdominales_), or below them _(Pisces_ s. _Pinnæ thoracicæ_), or in advance of them (_Pisces_ s. _Pinnæ jugulares_). They are generally narrow, composed of a small number of rays, the outer of which is frequently osseous. In some small groups of fishes, like the Gobies, the fins coalesce and form a suctorial disk. [Illustration: Fig. 6.--Salmo salar (Salmon), with abdominal ventral fins.] [Illustration: Fig. 7.--Mullus barbatus (Red Mullet), with thoracic ventral fins.] [Illustration: Fig. 8.--Burbot (Lota vulgaris), with jugular ventral fins.] For the definition of the smaller systematic groups, and the determination of species, the numbers of the spines and rays are generally of the greatest importance. This holds good, especially for the ventral rays, by the number of which the Acanthopterygian affinities of a fish can nearly always be determined. The numbers of the dorsal and anal rays generally correspond to the number of vertebræ in a certain portion of the spine, and are therefore constant specific, generic, or even family characters; but when their number is very great, a proportionally wide margin must be allowed for variation, and the taxinomic value of this character becomes uncertain. The numbers of the pectoral and caudal rays are rarely of any account. [Sidenote: Function of the Fins.]