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 orifice. 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 nerves 
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