SPECIES ALLIED TO MOTELLA. 29 



about midway of the body, as far below the lateral nerve as the fifth is 

 above it; and the rest occur at unequal intervals, at about the same height 

 as the lateral nerve. The location of these organs at different heights, and the 

 more perfectly preserved bilateral symmetry of those on the bead as compared 

 with those on the body, are points to which we shall recur again presently. 



The first and fourth pairs of sense-organs are very conspicuous, and are 

 easily seen from all sides, while the second and third can only be seen well 

 when the fish leans a little to one side. Tbe chief irregularities in number, 

 size, and position are found in the posterior sense-organs. The fourth pair 

 is in a favorable position for examining the membranous appendage, which 

 measures from .18 to .20 mm. in length by .02 mm. in width. The width 

 is nearly uniform, narrowing a little at the tip. It is perfectly straight, 

 immobile, and perpendicular to the general surface. 



As is well known, each of the sense-organs belonging to the body is 

 supplied with a branch of the lateral nerve, the entire course of which can 

 easily be traced in the living fish (Fig. 8).* In some of the young fishes of 

 this species from twenty-four to forty-eight hours old, the skin stands out at 

 some distance from the muscular axis, leaving a clear space, in which can be 

 seen with remarkable distinctness the lateral nerve, and its branches leading 

 to the organs of the lateral line. Such conditions were not observed in other 

 species, nor were they the rule in this. The individuals here figured ap- 

 peared perfectly health}-, and in all other respects normally developed. In 

 such specimens, nerves (posterior roots of cranial nerves) could be traced 

 to each pair of organs on the head except the first. The origin of these 

 nerves could not, of course, be determined by surface examination ; but 

 judging from their direction, and from what is now known of the innervation 

 of these sense-organs in Elasmobranchs, t we may assume that they repre- 

 sent the oculomotor, trigeminal, facial, and glossopharyngeal (which last. 

 according to Mayser, ^ belongs properly to the vagus group). Thus, to the 

 differences before mentioned between the sense-organs of the head and those 

 of the trunk we have to add a more fundamental distinction in respect to 

 nerve supply, each pair of the former being innervated by branches from a 

 corresponding pair of segmental cranial nerves, while those of the latter are 



* Ryder thinks that, tlieso organs are radi supplied by separate nerves coming directly from tbo spinal 

 cord (Embryograply of Osseous Fishes, p. 55). 



f Van Wijlie. "Ueberdie Mesodermsegmente and die Entwickelung der Nerven des Selachierkopfes," 

 Amsterdam, 1882. 



+ Zeitsch'r. f. miss. Zoo! , XXXVI., pp 303 304. 



