Branch I. and Class CYCLOSTOMATA. 



THE Cyclostoraes are the lowest of existing Craniates. They 

 form a small group of marine and freshwater animals, widely 

 distributed, but .containing comparatively few genera and species. 

 Like most isolated remnants of ancient stocks, the surviving 

 members are very specialised, and they appear to be also somewhat 

 degenerate. This, together with the absence of palaeontological 

 evidence, renders the interpretation of their structure very difficult. 



rrv. 



FIG. 



A, Petromyzon fluviatilis, L., the river Lamprey or Lampern. 13, Bddlostoma Dor,ibeyi, Lac. 

 C, Mi/xine glutinosa, L., the Hag-fish. ?>.o, branchial opening; o./, caudal fin; d, cloacal 

 aperture ; d.f 1 and d./ 2 , first and second dorsal tin ; e, eye ; g.s, gill-slit ; in. mouth ; w.s, 

 mucous sac ; n, nostril ; p.f, preanal fin. 



To Johannes Miiller we are indebted for an excellent account 

 of the anatomy of the Cyclostomes [306] ; many important additions 

 have been made by Fiirbringer [144], Parker [322], Schneider 

 [389], Dohrn [IHo, 116, 117], Cole [83], and numerous other 

 authors. 



The body is elongated and eel-like in shape, with an anterior, 

 almost terminal, mouth, and a median dorsal and ventral fin-fold 

 continuous round the tip of the tail (Fig. 1 5). No biting jaws are 

 present ; but from the floor of the buccal cavity protrudes a so-called 

 tongue, which is worked backwards and forwards so as to rasp the 



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