VERTEBRATE ANIMALS . 215 



very primitive arrangement indicating clearly a metameric segmenta- 

 tion. They are separated by thin fibrous sheets of connective tissue, 

 the myosepta or myocomata, to which the individual fibres lying in 

 the longitudinal direction are attached at each end. The lateral 

 line, which is partly embedded in a thick horizontal myoseptum, 

 separates the muscle segments into a dorsal or epiaxial series of 

 broader and a ventral or hypaxial series of narrower myotomes. 

 In the region of the eye, of the jaws and of the fins the fibres are 

 bound in bundles to form discrete muscles more like those of Rana. 



Endoskeleton. 



As in the frog, the endoskeleton may be divided for descrip- 

 tion into axial and appendicular portions. The axial part is com- 

 posed of the vertebral column and the skull, which in its turn is 

 formed by the cranium and a somewhat complex visceral skeleton. 

 The appendicular skeleton consists of the median fin supports, and 

 the pectoral and pelvic girdles with their corresponding limbs. 

 The endoskeleton as a whole is remarkable in that it always, even 

 in the adult, remains in a cartilaginous condition, no true bone ever^ 

 being developed although it is much strengthened by calcification, 

 i.e. the deposition in it of calcium salts in certain parts of the vertebral 

 centra. 



The vertebral column is a very primitive one, and composed of a 

 long series of vertebrae whose constituent parts are not nearly so 

 completely fused as in Rana, and so can be recognised as separate 

 pieces. It contains quite considerable remains of the notochord 

 in the adult. The centra are short stout cylinders with a deep 

 conical hollow at each end, a condition known as amphiccelous, and 

 one, it will be remembered, that is retained in the eighth vertebra 

 of the frog. The internal faces of the two hollows are lined with 

 calcified cartilage and communicate with one another by a small 

 central hole, the cavities of the hollows and the central holes being 

 filled with the persistent notochord. The centra are firmly bound 

 together by tough fibrous bands of tissue, the intervertebral ligaments, 

 which pass externally over the notochord. Dorsally they bear two 

 sets of cartilaginous plates which constitute the neural arches, and 

 together with a third set form a closed vertebral canal in which lies 

 the spinal cord. From the latero-dorsal aspect of the centrum arises a 

 pentagonal plate, the vertebral neural plate, which is only about half 

 the length of the centrum. Filling in the interstices between these 

 are hexagonal intervertebral neural plates, lying above the inter- 

 vertebral ligaments and so completing the neural arches. Fitted 

 into the notches left on the dorsal edges of the plates is a third series, 

 a row of unpaired, somewhat wedge-shaped cartilages, the neural 



