XIX] CLASSIFICATION 449 



backward into the alimentary canal just as it does in Amphi- 

 oxus. 



The tubular suctorial stomodaeum is represented by a hood-like 

 upper lip and a distinct short under lip, and when the mouth is 

 contracted the velum is produced into tentacles just as in the 

 Urochorda and in Amphioxus. The lateral eyes are exceedingly 

 rudimentary, but there is a large pineal eye, and the nasal sac 

 has a median septum. 



(ii) The MYXINOIDEA -are characterised by the persistent connec- 

 tion of the pituitary body with the stomodaeum, so that there is a 

 tube leading from the nasal sac to the mouth. There are eight ten- 

 tacles called barbels at the sides of the mouth, and there is no 

 special oesophagus distinct from the rest of the gullet. The skin has 

 a double series of mucous glands placed at the sides of the body, and 

 so much mucus can be thrown out that a large amount of water can 

 be rendered semi-solid. The intestine has no spiral valve. The 

 Myxinoidea are the animals known as Hag-fish. They adhere to fish 

 on whose flesh they feed, but, unlike the Lampreys, they can 

 actually burrow into their victims so that the stomodaeal region is 

 completely buried. In connection with these habits the stomodaeal 

 region is enormously elongated, and the eyes remain in a rudimentary 

 condition, whilst the gill openings are pushed very far back. 



The Myxinoidea include two genera, Bdelkstoma and Myxine. 

 In the former, which is a genus inhabiting the southern Atlantic 

 and Indian Oceans, the gill-sacs are seven in number on each side 

 and open separately; in Myxine, on the other hand, each external 

 opening of the six gill-sacs is drawn out into a long tube, and the 

 tubes of each side curve back and unite to open by a common 

 atrial pore placed so far back that the animal can insert almost half 

 its length into the body of its victim- without interfering with its 

 breathing. The branchial basket is vestigial in this genus. The 

 portal vein is rhythmically contractile, and thus constitutes an 

 accessory heart for the purpose of forcing venous blood through the 

 capillaries of the liver. Myxine is common on both the Atlantic 

 and Pacific coasts of North America and on the European coast. 



DIVISION II. GNATHOSTOMATA. 



The great division of the Gnathostomata includes all the remain- 

 ing Craniata, arid is characterised by the development of definite 

 visceral arches, jaws and paired limbs. The visceral arches 

 s. & M. 29 



