86 VERTEBRATES WITHOUT JAWS iv. 5 



muco-cartilage, which is an elastic material serving more as an 

 antagonist to the muscles than for their attachment. 



The notochord remains well developed throughout life as a rod 

 below the nerve-cord. It consists of a mass of large vacuolated cells, 



Fig. 49. Transverse section through notochord of lamprey. 

 c. cells; s. sheath. (After Krause.) 



anc 



Fig. 50. Lateral view of skeleton of head and branchial arches of Petromyzon. 



ac. auditory capsule; adc. antero-dorsal cartilage; anc. annular cartilage; ha. branchial 

 arch; bd. basidorsal; hac. hyoid cartilage; hbc. hypo-branchial rod; lit. horizontal bar; n. 

 notochord; ?ic. nasal capsule; oc. orbital cartilage; per. pericardial cartilage; pdc. postero- 

 dorsal cartilage; pic. posterolateral cartilage; st. styliform cartilage; /. tendon of tongue; 

 II-X, cranial nerves. (After Parker.) 



enclosed in a thick fibrous sheath (Fig. 49). The rigidity of the whole 

 rod depends on the turgor of the cells and it often collapses com- 

 pletely in fixed and dehydrated material (Fig. 59). No doubt in life it 

 serves, like the notochord of amphioxus, to prevent shortening of the 

 body when the myotomes contract. 



The notochordal sheath is continuous with a layer of connective 

 tissue, which also surrounds the spinal cord and joins the myocom- 



