CHAPTER II 



MYXINOIDS OR HAG-FISHES 



Characters. Habits. Mode of feeding. Life-history. Variability. Adapta- 

 tions in struggle for existence. Classification. 



THE Hags or Myxinoids are the simplest Craniates, and 

 have many remarkable peculiarities. The body is 

 eel-like and there is a row of large mucous glands on 

 each side. Above the jawless mouth there is an unpaired 

 nostril or naso-pituitary opening which leads into the buccal 

 cavity and is the channel by which the water passes in on its 

 way to the gill-pouches. Beside the mouth and the " nostril " 

 there are four pairs of short tentacles or barbules. There is a 

 single horny tooth on the roof of the mouth and on the floor 

 there are two pairs of comb-like tooth-plates which are worked 

 by a powerful muscular piston or " tongue," the huge develop- 

 ment of which has pushed the heart and gill-sacs unusually far 

 back. The gill-sacs open directly into the pharynx, and directly 

 or indirectly to the exterior. In Bdellostoma they open directly 

 to the exterior, in Myxine each is provided with an efferent 

 canal, and the canals of either side open together on the ventral 

 surface. In Paramyxine there is an interesting intermediate 

 arrangement. There are only a few small cartilages corre- 

 sponding to the lamprey's elaborate branchial basket. The 

 brain is poorly developed, notably as regards cerebrum and 

 cerebellum. The eyes are degenerate, without lens or muscles, 

 and do not reach the surface. There is only one semicircular 

 canal in connection with the ear, but it perhaps corresponds to 

 two fused together. The Myxinoids are the only normally 

 hermaphrodite vertebrates. The eggs are large and have horny 

 shells with adhesive threads ; as there is much yolk they divide 

 partially. So far as is known the development is direct with- 

 out larval metamorphosis. Myxinoids are widely represented in 

 the seas of both hemispheres. 



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