482 MANUAL OF ZOOLOGY. 



apertures in the form of slits or holes on the side of the neck 

 (fig. 260, B). The reproductive organs are ductless, and the 

 generative elements are shed into the abdomen, whence they 

 escape by an abdominal pore. 



The Lampreys are, some of them, inhabitants of rivers ; but the great 

 Sea-lamprey (Petromyzon marinus) only quits the salt water in order to 

 spawn. The mouth in the Pettomytonida is a circular cartilaginous ring, 

 formed by the amalgamation of the palatine and mandibular arches, and 

 carrying numerous teeth and small tubercles. The tongue is armed with 

 a double series of small teeth, and acts like a piston, enabling the animal 

 to attach itself to stones and rocks. There is no air-bladder. The body 

 is cylindrical, compressed towards the tail, and destitute of scales. The 

 skeleton consists of a series of cartilaginous rings without ribs. The young 

 Petromyzon undergoes a metamorphosis, being so unlike the parent that a 

 new genus (Ammocates) was originally founded for its reception. 



In the Myximda the mouth is circular and membranous, with eight cirri. 

 The palate carries a single fang, and the tongue is armed with a double 

 row of small teeth on each side. There may be seven branchial apertures 

 on each side (Heptatrema), or the branchial pouches open into a common 

 tube on each side, and each of these terminates in a distinct aperture 

 situated under the heart on the lower surface of the body (Myxine or Gas- 

 trobranchus}. The Hags pour out so much mucus through the lateral line 

 that they can surround themselves with jelly; hence the name of the com- 

 mon species (Myxine glutinosa). The Glutinous Hag is a native of the 

 North and British seas, and is chiefly found in the interior of the Cod and 

 Haddock (often five or six individuals in one fish). 



CHAPTER LIU. 

 TELEOSTEI. 



ORDER III. TELEOSTEI. This order includes the great ma- 

 jority of fishes in which there is a well-ossified endoskeleton, 

 and it corresponds very nearly with Cuvier's division of the 

 "osseous" fishes. The Teleostei are defined as follows : The 

 skeleton is usually well ossified ; the cranium is provided with 

 cranial bones ; and a mandible is present ; whilst the vertebral 

 column almost always consists of more or less completely ossified 

 vertebra. The pectoral arch has a clavicle; and the two pairs of 

 limbs, when present, are in the form of fins supported by rays. 

 The gills are free, pectinated or tufted in shape ; a bony gill-cover 

 and branchiostegal rays being always developed. The branchial 

 artery has its base developed into a bulbus arteriosus ; but this 

 is never rhythmically contractile, and is separated from the ventricle 

 by 110 more than a single row of valves. 



