FINS. 185 



and the hinder one the uiinogenital aperture. The genital 

 aperture may, however, in some forms be separate from and in 

 front of the urinary. 



The median fins are subject to considerable variation. 

 There are usually two dorsal fins, a caudal and an anal fin, but 

 there may be a single dorsal fin continuous round the tail with 

 the anal, or there may be a series of finlets in place of the dorsal 

 fin, and the anal may be multiple or absent. The pakedfins are m 

 two pairs, the 'pectoral and pelvic (sometimes called ventral). 

 Of these the pelvics are small, vary considerably m position 

 and are sometimes absent (fishes which live in mud) ; they may 

 be behind the pectoral on the abdominal surface in which case 

 they are said to be ahdominal in position, or they may be below 

 the pectorals {thoracic) or in front of them {jugular). In some 

 cases they coalesce and form a suctorial organ {Gobies) and in 

 some Blennies they are adapted for walking. The pectorals 

 are usually close behind the gill-opening and vary much in size. 

 They assist the fish in balancing and enable it to execute back- 

 ward movements. In some forms {Periophthalmus, Trigla 

 Lophiu^, etc.) the pectoral fins are used for walking, and in the 

 flying fish {Exocoetus) they are enlarged to form parachute-like 

 organs. All the fins are supported by osseous fin-rays 

 (dermotrichia) which are for the most part jointed and fiexible 

 (sometimes they branch peripherally), but in some cases 

 (Acanthopterygian fishes) more or fewer of the anterior der- 

 motrichia in the dorsal (anterior dorsal if there are two), anal, 

 and pelvic fins are unjointed and generally stiffened. Such 

 dermotrichia are called spines. The caudal dermotrichia are 

 always jointed, as are all the dermotrichia in the Malacoptery- 

 gian fishes. In some Malacopterygian fishes the posterior 

 dorsal fin is without jointed dermotrichia, but contains adi- 

 pose tissue only and delicate unjointed horny dermotrichia 

 resembling the embryonic dermotrichia ( actinotrichia) : 

 this is the adipose fin of certain Siluridae, Salmonidae, etc. 

 The dermotrichia of the ventral part of the caudal fin are 

 carried by the haemal arches, those of the other unpaired fins 

 by special skeletal rods — the somactids (p. 54) — lying in the 

 median fibrous septum separating the dorsal parts of the muscles. 

 These somactids of the Teleostean unpaired fin are often called 

 interspinous bones. 



