171 



orbital canal has o pores and 4 sense organs, the left supra- 

 orbital '3 pores and 2 sense organs, and the supraorbital 

 commissure one median pore and a pair of sense organs. 

 Only the two extremities of the left svipraorbital canal are 

 present, the whole of the intermediate portion having 

 aborted with the corresponding part of the left frontal. 

 The first two pores and first sense organ of the left supra- 

 orbital appear to correspond to the same on the right. 

 The whole supraorbital system is supplied by the two RR. 

 ophthalmici superficiales vii. 



Besides the sense organs situated in the sensory 

 canals, or canal organs, there are two other series of naked 

 sense organs situated on the skin. These are known as 

 Pit Organs and Terminal Buds. The former belong to 

 the lateral line system, and are innervated by one or more 

 of the four lateral line nerves. The latter are quite dis- 

 tinct from the lateral line organs, and are innervated by 

 an entirely separate system of nerves, the Ramus lateralis 

 accessorius ( = R. lateralis trigemini), belonging to the 

 communis system. The terminal bud system has been 

 reduced in the Plaice. 



2.— The Nose (Fig. 25).* 



The olfactory organ of both sides is described as if 

 viewed from above, and not from the side. The external 

 apertures of the nose, two on each side, are not symmetri- 

 cally placed, and those of the left side are situated almost 

 on the apparent mid-dorsal line of the body, immediately 

 to the left of the anterior border of the left eye. The 

 two apertures or nostrils of each side are sometimes called 

 anterior and posterior nares, but the latter term is 

 obviously inadmissible. 



* For the olfactory organs of the Pleuronectidae, see Kyle, Jour. Linn. 

 Soc, xxvii., and 18th Ann. Rep. Fish. Board Scotland, 1899, Part iii. 



