XIX] SKELETON 447 



completely constricted off from the remainder of the gullet and is 

 known as the oesophagus, though this word is used in a different 

 sense from that in which it is used in the case of the Gnathostomata. 

 The lower part of the gullet which communicates with the gill-slits 

 in these cases ends blindly behind and is called the respiratory 

 tube. 



The hinder part of the alimentary canal is a nearly straight 

 tube, the spiral valve having a very slight deviation from a straight 

 course. There is no dilatation of any kind in its course. The 

 large liver empties its secretion by the bile-duct, which opens into 

 the intestine a short distance behind the branchial region. 



The skull consists of the simplest elements, viz. the trabeculae, 

 with a wide hole for the infundibulum, and the parachordals, 

 which form only a slender arch over the hinder part of the brain, 

 but develop also a low side wall throughout their extent with which 

 the simple auditory capsule is fused. There is a loop of cartilage 

 attached to each parachordal termed the subocular arch and 

 a curved bar of cartilage behind this which is attached above to the 

 skull and passes down along the pharynx. In Myxiuoidea the latter 

 encircles the pharynx and unites with its fellow in the mid-ventral 

 line and joins the cartilage supporting the tongue. This second 

 arch is termed the hyoid, and with some plausibility both it and 

 the subocular bar are regarded as degenerate remains of the first 

 two visceral arches of other Craniata. The nasal capsule is re- 

 presented by cartilage stiffening the nasal tube. The brain is 

 remarkable for having a thin membranous roof except just at the 

 front end of the hind-brain where a narrow band of nervous, matter 

 represents the cerebellum. 



The only fins present consist of a fringe of skin similar to that 

 found in Amphioxus surrounding the hinder end of the body in the 

 vertical plane. This fringe is divided by a notch into an anterior 

 (or dorsal) and a caudal fin. The dorsal fin is supported by 

 cartilaginous rays situated above which represent the neural arches 

 which protect the spinal cord ; the caudal fin has, in addition to 

 these, rays situated below which are to be regarded as haemal arches. 

 A caudal fin of this description, which the notochord divides into 

 two equal lobes, is called diphy cereal. 



Besides the neural ar-ches (13, Fig. 220) and small haemal arches 

 in the tail no other cartilage is developed in connection with the 

 axial skeleton, the notochord with its thick fibrous sheath persisting 

 unchanged throughout life. 



