THB LOWER VERTEBRATA. 171 



the frog, but without teeth. In some urodele amphibians 

 the vomers and the splenials of the lower jaw are known to 

 arise by the fusion of small denticles. These facts seem to 

 point to stages in the fusion of placoid bases, and their 

 withdrawal from the surface to become incorporated with 

 the cranial apparatus ns membrane bones, a process entirely 

 completed in the mammalian type. 



3, Sensory Tubules. Scattered over the snout, especially 

 on its ventral surface, are numerous fine pores. These are 

 the openings of long tubes, filled with a jelly. Internally 

 they end in dilatations which resemble the ampullae of the 

 ear, and whose minute structure and nerve-supply show 

 them to be sense-organs : they evidently are concerned with 

 a sense possible only to aquatic animals, for similar tubes 

 occur in the tadpole and are lost in the adult frog. The 

 most reasonable suggestion is that they are organs of a 

 p?-m?'e-sense, which must be needful in animals that swim 

 in fish-like fashion. In most fishes similar but shorter 

 tubes are arranged in a row on either side, forming the 

 lateral line which is so conspicuous in the salmon, for 

 example. But in the dogfish the lateral line is very feebly 

 developed, though it can be made out in transverse sections. 



4. Transverse Sections. In transverse sections (figs. 88 

 and 89), the most obvious difference from frog and rabbit 

 is the great development of the body-muscles, due to the 

 swimming habit. These muscles, if dissected from the side, 

 are seen to be metallic rically segmented to a much more 

 complete extent than in frog and rabbit. Each muscle- 

 segment is called a myomere or myotome, is shaped like a 

 V, the point directed forwards, and the ventral limb longer 

 than the dorsal. Owing to this shape, parts of several 

 myomeres are seen in a transverse section. At their dorsal 

 ends the myomeres alternate with the vertebrae, i.e. one 

 myomere is attached partly to one vertebra and partly to 

 the next, while each vertebra is pulled by parts of two 

 myomeres on each side. In this way the sinuous movement 

 characteristic of swimming is rendered possible. 



6. The Coelom extends farther forward than in the frog. 



