MUSTELUS LiEVIS. lOl 



outer surface. There were, however, four short tubules 

 leading outward from the canal between certain of the organs 

 and ending blindly, these tubules thus being in process of 

 abortion. This plainly indicates that other tubules, related 

 to the other organs, have here wholly aborted. This partial 

 or complete abortion of these seven tubules is most un- 

 questionably due to the near presence of the spiracular canal, 

 though why, for this reason only, the}^ should have become 

 aborted instead of being retained as short tubules openino- 

 along the anterior edge of the spiracle, is not evident. 



Wright (70) describes in the so-called anterior diverti- 

 culum of the spiracular cleft of Mustelus what he considers, 

 under some reserve, as a sense organ belonging to the category 

 of lateral sensory organs, but said to be of hypoblastic instead 

 of epiblastic origin. It is said by him to be innervated by 

 fibres derived from the pretrematic branch of the facialis. My 

 observations lead me to believe that the organ here referred 

 to is innervated, on the contrary, by a branch of that one 

 of the three branches above described that innervates organs 

 87 to 91 infi-a-orbital. As the observations on which this 

 conclusion is based i-equire some explanation, it will be dis- 

 cussed in a later section after the spiracular cleft and certain 

 related structures have first been described. The fact that 

 the innervation of this spiracular organ shows that it is 

 quite intimately related to, if it does not actually belong 

 to, the infra-orbital line should, however, be mentioned here. 



Supratemporal Cross-commissure and Lateral 

 Canal of the Body. 



The supratemporal cross-commissure arises, on either side, 

 from the hind end of the otic section of the main infra-orbital 

 canal, and curving slightly forward crosses the middle line 

 of the head, then curves backward, and joins the infra-orbital 

 canal of the opposite side. The commissure passes immediately 

 posterior to the pores of the endolymphatic ducts, those pores 



