ioo MORPHOLOGICAL TYPES 



the skull and is counted as the ist spinal nerve ; it runs to the 

 muscles beneath the tongue which actuate the " hyoid " plate 

 for the purpose of breathing. The spinal nerves to the limbs 

 are grouped, forming the brachial and sciatic plexus for the 

 fore and hind limb respectively. 



As in the dogfish, there are sympathetic nerve-chains on 

 each side of the dorsal aorta. They continue forwards 

 accompanying the internal carotids into the head. They join 

 the sympathetic ganglia to one another, each receiving in 

 addition a ramus communicans from its corresponding spinal 

 nerve. 



Sense-organs. — The lateral-line sense-organs have already 

 been mentioned. The ears are in a degenerate condition in 

 Triton, for although they appreciate vibrations in air, i.e. 

 sound, their structure is not typical of land- vertebrates, and 

 will not be considered here. It may be mentioned, however, 

 that the tympanic cavity which is characteristic of the ears of 

 other Tetrapods and which is homologous with the spiracular 

 slit of the dogfish, is not developed ; and that the ear-drum 

 or tympanic membrane is also absent. 



The lens of the eye is attached to a protractor lentis muscle, 

 contraction of which increases the distance between the lens 

 and the retina and accommodates the eye for near vision. 

 There is a retractor bulbi muscle which pulls the eyeball in, 

 and depresses the roof of the mouth, which action assists in 

 the process of swallowing. 



Mesoderm and Ccelom. — An important feature is that the 

 wall separating the pericardium from the perivisceral ccelom 

 is very thin and membranous, and unlike the stiff partition 

 present in the dogfish and Gadus. The amphibian condition 

 is already foreshadowed in Ceratodus, and it results in the 

 fact that the heart and pericardium project back into the 

 perivisceral cavity, ventral to the gut. 



The first three somites give rise to the eye-muscles. The 

 4th disappears during development, and the 5th produces 

 muscle-fibres which persist. 



Ductless Glands . — The thyroid arises from a downgrowth 

 from the floor of the pharynx and afterwards divides into two, 



