SECTION IV 



GYGLOSTOMATA OR 

 MARSIPOBRANCHS 



CHAPTER I 



INTRODUCTORY 



General structural characters and definition. 



THE Cyclostomes or " Round-Mouths," including hags 

 (Myxinoids) and lampreys (Petromyzonts), form a 

 primitive class of vertebrates. They differ from fishes, 

 (i) in having round suctorial mouths, without definitely de- 

 veloped jaws, (2) in the absence of paired fins, (3) in the 

 peculiar purse-like gill-pouches, and (4) in showing many unique 

 features such as the unpaired nasal sac. As the distinction be- 

 tween animals without jaws and animals with jaws is anatomic- 

 ally deep, the Cyclostomes stand apart from the Gnathostomes, 

 which include fishes and all the other vertebrates. They repre- 

 sent a remnant of an ancient stock, and like other types which 

 may be so described, AmpJiioxus, for instance, they show a 

 combination of simplicity and specialisation. Their wide geo- 

 graphical representation may also be interpreted in relation to 

 their ancient origin. 



The Cyclostomes are marked by the following general 

 characters : the eel-like form, the smooth scaleless skin ; the 

 absence of paired fins or girdles ; the suctorial mouth, with 

 horny teeth, without definitely developed jaws ; the cartilaginous 

 skeleton and persistent unsegmented notochord ; the absence 

 of dermal fin-rays in the unpaired fins ; the single nasal organ 

 curiously combined with the pituitary sac ; the purse-like gill- 

 pouches; the straight intestine ; the absence of genital ducts. 



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