178 VERTEBRATA. 



The persistent notochord extends beyond the cerebro-spinal 

 axis, in all other vertebrates it stops behind the pituitary gland. 

 Some of the great blood-vessels are rhythmically contractile, but 

 the blood is colourless. Eespiration is effected by a ciliary mem- 

 brane lining the pharynx, which extends to nearly half the length 

 of the body, and is continued into a straight simple intestine. The 

 eyes are rudimentary, there is no organ of hearing, and there are 

 no scales. 



Until very recently only one species was certainly known, the 

 lancelet, AmpTiioxus lanceolatus ; it is found on the English coast, 

 and is probably an inhabitant of most seas. It is a small trans- 

 parent creature, with a delicate fin extending the whole length 

 of the back and passing round the tail to the anus. Pallas first 

 described it as a Limcuc. Peters has a second genus from Moreton 

 Bay, Epigonichtfiys ; it has no anal nor caudal fin, and the anal 

 aperture is median ; only one species is known E. cultellus. 



Amphioxus=Branchiostoma. 



Order IL MAESIPOBEANCHII. 



CYCLOSTOMI. MONORIIIXA. DERMOPTERI. 



Notochord persistent. Skeleton cartilaginous ; no ribs nor 

 limbs. Mouth suctorial, but without jaws. Gills sac-like, com- 

 municating externally by six or seven holes. No bulbus arte- 

 riosus. 



The body is eel-shaped, and the skin is without scales. The 

 eyes are either wanting or are very small. The mouth, longitu- 

 dinal when closed, circular when open, has flexible lips capable 

 of adhering to any smooth substance, with numerous small teeth 

 within. There is only one nasal opening. In Myxine the lips 

 are provided with 6-8 cirri, and its teeth are developed in the 

 median line of the palate. 



There is a distinct brain ; but the skull is without sutures, and 

 not separable from the vertebral column. There is no air-bladder. 

 The kidneys are well developed. 



The hag (Myxine glutinosa) is without eyes ; it bores into and 

 lives in the interior of other fish. The pride (Ammoccetes branchi- 

 alis} is the larval form of the river-lampreys (Petromyzan fluvia- 

 tilis and P. planeri) ; it is three years before the adult form is 

 acquired. Conodonts, supposed to belong to the Myxinidae, are 

 minute Palaeozoic tooth-like fossils. 



There are two families. Hyperotreta of Miiller are the Myxi- 

 nidse, and his Hyperoartia are the Petromyzontidse. 



