glossopharyngeal nerve IX 

 temporal canal 

 facial nerve (VII) root^ spiracle 

 supraorbital canal 



infraorbital canal 



oral canol 



jugal canal 



ventral-line organs 



Figure 13-27. Sensory-line system of Squo/us aconthias. (After Norris and Hughes, 1920) 



the preopercular division, extends down and forward to be- 

 come the mandibular division on the lower jaw. The ter- 

 minology here is confusing. The mandibular segment of the 

 hyoid arch line anteriorly joins the oral segment of the 

 mandibular arch line. The preopercular division ends be- 

 hind and below the spiracle without connecting with the 

 temporal portion of the main lateral line. The posterior pit 

 line, single or double, extends toward but not to the mid- 

 line in front of the endolymphatic pores. Some of the lines 

 are open grooves; these include the entire hyoid arch line, 

 the oral portion of the mandibular arch line, the posterior 

 tip of the jugal extension, and the lateral line posterior to 

 the spiracle. The others are closed canals opening to the 

 surface through irregularly spaced pores. 



There is a pit line from the spiracle down toward the 

 junction of the jugal and oral lines. The significance of this 

 pit line is not known, but it may be the satellite of the post- 

 orbital section of the mandibular arch line. Pit lines such 

 as this are not constant in position nor relationships, while 

 others, as remnants of canals, are more fixed in position. 



In most selachians, such as the dogfish (Figure 13-27), 

 the canal pattern is suggestive of that observed in CUamydo- 

 seiachus; however, the mouth is far back below a strongly 

 developed rostrum. Development of the rostrum has appar- 

 ently distorted the proportions of the canals. The infraor- 

 bital canal ends below the tip of the snout. (The canals of 

 either side meet and join for a short distance in Laemargus; 

 this fusion occurs well back from the ends of the canals just 

 in front of the mouth.) In Laemargus the supraorbital, infra- 

 orbital, and lateral lines come together at a point high on 

 the head. 



In most sharks the hyoid line is represented only by pit 

 organs (a case of canal reduction to a pit line); the oral 

 canal may be much reduced and without direct connection 

 with the jugal canal, which has a strong posterior extension. 

 The occipital canal lies behind the endolymphatic pores 

 and the canals of either side connect. This canal is also 

 called the supratemporal, aural, or commissural canal. A 

 short branch of the occipital canal may pass in front of the 

 endolymphatic pore; this is the posterior pit-line canal. The 

 sensory lines are canals in most sharks although the poste- 

 rior end of the lateral line may be a groove as in Squalus. 



The development of the sensory system oi Squalus has been 

 described. In the embryo a number of ectodermal placodes 

 give rise to separate supraorbital, infraorbital, jugo-oral, 

 distal and proximal hyoid, temporal, posterior pit lines, and 

 lateral line divisions (Figure 13-28). The adult pattern is 

 thus indicated at the time of appearance of the placodes. 

 Of interest is the fact that the jugo-oral and distal hyoid 

 placodes arise joined by the jugal extension. In development, 

 having the organs in a groove precedes the formation of 

 closed canals. The pit organs of the hyoid arch line are 

 formed by fragmentation of the placode. Each fragment 

 sinks down into the epidermis to form an organ. 



Holocepbatan The pattern observed in Hydrolagus is dis- 

 tmct from that of the shark, but suggestive of it (Figure 

 13-29). The hyoid and mandibular arch lines join the infra- 

 orbital below the eye. The first is the better developed. The 

 infraorbital line has a break in it and passes above (behind) 

 the nasal capsule. Assuming that this lower line represents 

 the infraorbital, the upper could correspond to the reflexed 



SENSORY ORGANS 



413 



