THE LATERAL LINE ORGANS. 27 



The canals are also named in accordance with the bones 

 containing them. Thus, the main canal has extra-scap- 

 ular and squamosal portions, the supra-orbital canal has 

 frontal, and nasal portions, the infra-orbital has post- 

 orbital and lachrymal portions, etc. 



/. — TJie Lateral Line of the Trunk. 



The family Atherinidae is characterized in the system- 

 atic works as lacking the lateral line. In Menidia the 

 lateral line canal is absent on the trunk, but is represented 

 by a row of very small lateral line organs innervated by 

 the r. lateralis vagi, one for each segment of the body. 

 Whether they extend the whole length of the body I have 

 not ascertained. They can be followed back from the 

 head by surface examination for only a short distance, 

 one on each scale. But the markings on the scales of 

 the lateral line series, i. e., the groove in the centre of 

 the scale in which the organ lies, can be recognized about 

 half-way back to the tail. Probably minute lateral line 

 organs extend still farther caudad. 



Merkel ('80) enumerates a number of teleosts which 

 have the lateral line developed in various degrees and 

 there are numerous other accounts in the literature of the 

 absence of the lateral line canals, the lines being repre- 

 sented by rows of naked organs. For example, Leydig 

 ('94, p. 30) states that in Leucaspius delineatus for the 

 caudal sixth of the body the lateral line canal fails, but 

 the lateral line nerve continues and supplies a series of 

 naked organs lying in a groove. The tendency for the 

 canal to fail to appear in the trunk is doubtless to be 

 correlated with the fact that in the ontogeny the trunk 

 canal closes later than the head canals (Allis, '89, Ley- 

 dig, '94). 



