CHAPTER XVI 



CYCLOSTOMATA (SYSTEMATIC) 



CLASS I. CYCLOSTOMATA 



The Cyclostomata, or, , as they are sometimes called, the 

 Marsipobrauchii, from the pouch-like, or rather sac -like, shape 

 of their branchial clefts, are divided into two orders, the first 

 comprising the " Hag-Fishes " or " Borers," and the second the 

 Lampreys. 



Order I. Myxinoides. 



The Hag-Fishes are probably the most primitive of all exist- 

 ing Craniates. The mouth is nearly terminal, and there is no 

 buccal funnel. The naso-pituitary involution communicates be- 

 hind with the oral cavity and functions as a channel for the 

 in-streaming water-current to the gilis. Four pairs of short 

 tentacles, supported by a special tentacular skeleton, are present 

 in relation with the mouth and the terminally -placed naso- 

 pituitary oriiioe. The gill-sacs open directly into the pharynx. 

 The branchial basket is but feebly developed, and at the most it 

 is only represented by small isolated cartilages in relation with 

 the external branchial apertures. The lingual apparatus is 

 remarkably developed. Besides the lingual teeth there is only a 

 single dorsal tooth in the roof of the mouth. The dorsal arcualia 

 are restricted to the tail, or they extend for a short distance only 

 into the trunk. A spiral valve is absent. There is a row of 

 mucus-secreting sacs along each side of the body. The brain has 

 no obvious cerebral hemispheres, nor a cerebellum. Only one 

 semicircular canal is present in the auditory organ. The eyes 

 are degenerate, and the usual eye-muscles with the cranial neryes 



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