J16 



SUB-CLASS MARSIPOBRAXCHII (CYCLOSTOMATA). 



doubt many individuals were then produced with imperfect visual organs. 

 Most of these naturally died out in competition with their more highly 

 endowed brethren, but in some cases compensating advantages in other 

 organs enabled them to hold their own in spite of their defective sight. 

 To hold that a free-living animal, and a myxinoid must after all be regarded 

 as such, can lose its eyes through disuse would seem to be an impossible 

 position. The absence cannot be considered as other than a disadvantage 

 to it. 



Fam. 1. Petromyzontidae (Hy- 

 peroartia), lampreys, nine-eyes. 

 With seven external gill apertures 

 on each side which lead into a sub- 

 pharyngeal tube opening anteriorly 

 into the pharynx and ending 

 blindly behind, and with a com- 

 plete branchial basket-work. The 

 suctorial mouth is surrounded by 

 a circular lip and is provided w r ith 

 horny, simple or multicuspid teeth, 

 without barbules. The single nasal 

 opening is in the middle of the up- 

 per side of the head, and the nasal 

 duct (pituitary sac) ends blindly 

 behind. Eyes are present. With 

 two dorsal fins, and a spiral valve in 

 the intestine ; gall-bladder absent. 

 The eggs are small, and there is a 

 prolonged larval stage in which 

 the young are known as Ammo- 

 coetes. Fresh waters and coasts 

 of the temperate regions of both 

 hemispheres. Petromyzon Art. 

 coasts and fresh-waters of the 

 northern hemisphere ; P. marinus 

 L., sea-lamprey ; P. ftuviatilis L., 

 river-lamprey. Ichthyomyzon Gir- 

 ard, west coast of North America ; 

 Mordacia Gray, without gular 

 pouch, coasts of Chile, south-east 

 Australia and Tasmania, entering 

 fresh-water to breed ; Geotria, Gray, 

 with gular pouch, rivers of Chile, 

 mal). south and south-east Australia and 



New Zealand ; Velasia Gray, with- 

 out gular pouch, is an immature 

 stage of Geotria. 



Fam. 2. Myxinidae (Hyperotreta). Nasal aperture single, at the anterior 

 end of the body ; the nasal duct (pituitary sac) opens posteriorly into the 

 pharynx and has cartilaginous rings ; mouth suctorial, without lips, with 

 barbules, with one median palatal tooth and two rows of lingual teeth ; 

 branchial openings far behind the head, lead directly into pharynx ; branch- 

 ial basket-work not present ; a series of mucous sacB on each side of the 

 body ; eyes hidden under the skin, and very imperfect, without lens or 



d.KC 



FIG. 61. Ventral view 

 of anterior end of 

 Bdellostoma forsteri 

 (after W. K. Parker) 

 br p. external aper- 

 tures of branchial 

 sacs ; d. oe c opening 

 of ductus oesopha- 

 geus cutaneus. 



FIG. 62. Myx- 



