192 



CHORDATA. 



rudimentary, and do not close on each other as do the jaws of other verte- 

 brates. This cyclostome condition is of value to the animals, as it aids 

 them in sucking on 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. 



The name Marsipobranchs refers to the form of the gills, usually six 

 or seven in number, but in Bdellostoma may be twelve or fourteen on either 

 side. Kach gill cleft consists of three parts, the gill sac (marsupium)^ 



FIG. 542. FIG. 543. 



FIG. S4 2 - Mouth of Pciromyzon murimis with horny teeth and tongue (from 

 ' ;enbaur). 



In;. 543. Gill apparatus of Myxine ^/utitiosa (after J. Miiller). a, atrium; ab, 

 "ill artery and gill arch; br, gill sac (the lines show the gills); br', efferent canal; c, 

 cesophageo-cutaneus duct; d, skin turned away; i, afferent gill canal; , oesophagus; 5, 

 mouth of atrium; v, ventricle of heart. 



which contains gills, and the afferent and efferent ducts (fig. 543). These 

 ducts develop separately, and may continue so (Bdellostoma), but in 

 J'ftnnnyzon the afferent ducts unite to a single tube which opens ventrally 

 iii the pharynx. In Myxine (fig. 543) the conditions are reversed, the 

 efferent canals of a side uniting to empty through a single external 

 opening. 



A third name, Monorhina, has been given, since these forms, in con- 

 trast to all other vertebrates, have an unpaired olfactory organ. The 

 single nostril, lying in the mid line of the head, opens into a nasal sac, from 

 the bottom of which a canal descends towards the roof of the mouth, end- 

 ing blindly in Petromyzontes (Hyperoartia), or penetrating the palate in 

 the My/ontes (Hyperotretia), so that an inner nasal opening (choana) into 

 the pharynx occurs. A paired olfactory nerve supplies the organ. 



