556 



CHORD AT A. 



to other animals. At the base of the dome-like mouth cavity is 

 the so-called tongue, which is the sucking apparatus, since it can be 

 drawn backwards like a piston (fig. 584). 



The name Marsipobranchs has been given on account of the 

 form of the gills, which are usually six or seven in number, but in 

 Bdellostoma may be twelve or fourteen on either side. Each gill 

 cleft consists of three parts, the gill sac (marsupium), which alone 

 contains gills, and the afferent and efferent ducts (fig. 585). These 

 canals arise separately, and may continue so (Bdellostoma), but in 

 Petromyzon the afferent ducts unite to a single tube which opens 

 ventrally in the pharynx. In Myxine (fig. 585) the conditions are 

 reversed, the efferent canals uniting to empty through a single 

 external opening. 



A third name, Monorhina, has been given, since these forms, 

 in contrast to all other vertebrates, have an unpaired olfactory 

 organ. The single nostril, lying in the mid line of the head,. 



FIG. 584. FIG. 585. 



FIG. 584. Mouth of Petromyzon marinus with horny teeth and tongue. (From Gegenbaur.)- 



FIG. 585. Gill apparatus of Myxine glutinosa. (After J. Muller.) a, atrium; 6, gill artery 



and gill arch : 6r, gill sac (the lines show the gills) ; for', efferent canal; c, cesophageo- 



cutaneus duct; rf, skin turned away; t, afferent gill canal; o, oesophagus; s, mouth 



of atrium ; v, ventricle of heart. 



opens into a nasal sac, from the bottom of which a canal descends 

 towards the roof of the mouth, ending blindly in Petromyzontes 

 (Hyperoartia), or penetrating it in the Myzontes (Hyperotretia), 

 so that an inner nasal opening (choana) occurs. A paired olfac- 

 tory nerve supplies the organ. 



Sub Class I. Myzontes (Hyperotretia). 



Semiparasitic cyclostomes with cirri around the mouth, very primitive 

 nephridia, right and left rows of slime sacs, eyes rudimentary (lens, sclera, 



