SELACHII. 157 



lips, and surrounded by labial processes ; eyes hidden beneath the skin. There 

 is an opening from the nasal cavity into the mouth through the posterior part 

 of the palate. The branchial pouches open to the exterior either by a common 

 ventral aperture on each side (Myxine, Gastrobranchus) or by seven apertures 

 on each side or asymmetrically by six apertures on one side and seven on the other 

 (Bciellostoma). Marine. Myxineglutinosa~L. (fig. 599). Bdellostoma heptatrema 

 Joh. Miill., found at the Cape. 



Fam. Petromyzontidae. Lampreys Nine-eyes. With seven external gill 

 slits on each side of the neck, and a common internal branchial passage 

 which opens anteriorly into the pharynx. The nasal cavity ends blindly 

 The round mouth, without labial processes, with fleshy lips, which can be 

 approached so as to leave a slit-like opening. Petromyzon marinus L. 

 Lamprey, two feet in length, ascends rivers with the Shad, in the spring, 

 to spawn. P. fiuviatilis L., Kiver Nine-eye (fig. 602 a"). P. Planeri Bloch, 

 small river Nine-eye, with Avimoccetes branchialis as larva. It attains a 

 length of 6 to 6 inches. 



Spl 



FIG. 603. Acanthias vulgarie Spl, spiracle ; K*., gill slits. 



\ Order 3. SELACHII* (CHONDROPTERYGII). 



Cartilaginous Fishes with large pectoral and pelvic fins, with 

 transverse ventr ally -placed mouth, usually with five (rarely six or 

 seven) pairs of branchial pouches and branchial slits. They have a 

 muscular conus arteriosus which contains several rows of valves. The 

 intestine has a spiral valve. 



The Selachians differ strikingly in their outward appearance from 

 all other Fishes (fig. 603), and present even among themselves great 

 variations. The form and position of the mouth, which is a broad 

 transverse slit placed on the under surface of the snout, is an impor- 

 tant distinguishing character. The skin usually contains a number 

 of bony granules (ossified dermal papillae, placoid scales), and obtains 



* Compare Joh. Miiller and J. Henle, " Systematische Beschreibung der 

 Plagiostomen, " mit 60 Steindrucktafeln. Berlin, 1841. 



Fr. Leydig, " Beitrage zur mikroskopischen Anatomic und Entwickelungs- 

 geschichte der Eochen und Haie." Leipzig, 1852. 



C. Gegenbaur, " Untersuchungen zur vergleichenden Anatomic der Wirbel- 

 thiere." Leipzig, 1872. 



F. M. Balfour, " A monograph on the development of Elasmobranch Fishes." 

 London, 1878. 



C. Hasse. " Das natiirliche System der Elasmobranchier." Jena, 1879. 



