492 



CHORDATA. 



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

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

 them in sucking on to other animals. At tlae base of tlae dome-like 

 month cavitv is the so-called tongue, which is the sucl-dng apparatus, 

 since it can be drawn backwards like a piston. 



The name ^Marsipobranchs refers to the form of tlie gills, usually sis 

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

 side. Each gill cleft consists of three parts, the gill sac {i)iars:ipiuiii), 



Fig. 542. Fig. 543. 



Fig. 542. — ilouth of Pclromyzou mariinis with horny tcclh and tongue (from 

 Gegenbaur). 



Fig. 54^ — GUI apparatus of My.xiiic f;liiti)iosa latter J. ^liiUer). a, atrium; ah, 

 gill arterj- and gill arch; br, gill sac (the lines show the gills); br', elTerent canal; r, 

 oesophageo-cutaneus duct; </, skin turned away; /, atTerenl gill canal; 0, cesophagus; .^, 

 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 (BdcUos/oma), but in 

 Pclromyzon the alferent ducts unite to a sittgle tube which opens veutrally 

 in the pharyn.x. In Myxiiic (lig. 543) the coirditioris are reversed, the 

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

 opening. 



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

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

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

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

 ing blindly in Petromyzontes (H^qaeroartia), or penetrating the palate in 

 the Myzontes (H^i^erotretia), so that an inner nasal opening (choana) into 

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



