404 CYCLOSTOMATA. 



the skin, and the degeneracy of the eye may be associated 

 with the hag's mode of life. 



Alimentary System. — The mouth is suctorial. There is a 

 median tooth above, and two rows of teeth are borne on 

 each side of the muscular tongue. These teeth are "horny," 

 but sharp. Into the mouth, just in front of a fringed velum 

 which separates it from the pharynx, the nasal sac opens. 

 Thus water passes from the nostril into the pharynx. It 

 may be, as Beard suggests, that this passage is a persistent 

 " old mouth." From the gullet open six respiratory pouches, 

 each of which has an efferent tube, but the six efferent tubes 

 of each side have a common exhalent orifice. The gut is 

 straight and uniform, with longitudinal ridges internally, 

 with a two-lobed liver and a gall-bladder, but without the 

 usual pancreas. 



Respiratory System. — Water enters by the nasal sac, passes 

 into the pharynx, down the gullet, into the six pairs of respira- 

 tory pouches, out by their efferent tubes at a single aperture 

 on each side. The respiratory pouches have much plaited 

 internal walls, on which the blood is exposed. 



Vascular System. — The blood contains nucleated red- 

 corpuscles. It is collected from the body in anterior and 

 posterior cardinals, passes through a sinus venosus into the 

 auricle of the heart, thence to the ventricle, thence along 

 a ventral aorta which gives off arches to the respiratory 

 pouches. From these the purified blood passes dorsalwards 

 m efferent branchial vessels, which unite posteriorly to form 

 the dorsal aorta, while from the most anterior a branch goes 

 to the head. 



Excretory System. — The segmental or archinephric ducts 

 remain unsplit, and the kidney or nephridial system is 

 represented by a series of small segmental tubules attached 

 to the ducts. According to some the pronephros or fore- 

 kidney persists, though apart from the functional meso- 

 nephros ; according to others it disappears. The segmental 

 ducts are said to end much in the same way as they do in 

 the lamprey. 



Reproductive System.— Myxine is hermaphrodite, but the 

 ova and spermatozoa are formed at different periods in its 

 life. The reproductive organ is simple, unpaired, and 

 moored by a median dorsal mesentery. Owing to the large 



