LATERAL LINE ORGANS. 



123 



Let us now consider the several lines of canal organs as they appear in ostra- 

 coderms and primitive vertebrates. 



In the ostracoderms, they undoubtedly occur in the most primitive condition 

 known in the adult of any vertebrate-like animal. 



In Tremataspis (Fig. 236), the organs were apparently located in short, shal- 

 low surface grooves; in Bothriolepis (Fig. 247), in continuous open grooves. 

 When expressed in a simple diagrammatic form, the sensory grooves of the ostra- 

 coderms appear to originate in the occipital region and to radiate from it in the 

 following lines: There is a main suborbital (Fig. 89, B.}, i.o.L, continued for- 

 ward as the rostral line, r.L, in front of the olfactory organs. In Bothrio- 

 lepis, a branch line arises from it and extends hsemally over the surface of the 



A. B. C. D. 



FIG. 89. Schematic figures showing the location of the lateral and median eye, olfactory, auditory, gusta- 

 tory, and canal organs, in the arachnids, ostracoderms, arthrodiri, and primitive vertebrates. All figures seen 

 from the neural surface. 



premaxillae. A mandibular line is not recognized in any ostracoderm, prob- 

 ably owing to the small size of the mandibles. There is no true supra-orbital 

 line, probably owing to the median location of the lateral eyes, although the 

 short-post orbital line of Tremataspis and the longer one in Bothriolepis possibly 

 represent the proximal end of such a line, s.o.l. 



The orbital line appears to be continuous with the lateral line of the branchial 

 region and of the trunk, by means of short glosso-pharyngeal sections, g.p. Judg- 

 ing from the embryological conditions in vertebrates, this section represents a 

 separate line, supplied solely by the glosso-pharyngeal nerve. The main lateral 

 line extends along the branchial region and in Bothriolepis may be traced for a 

 short distance on to the trunk. There are two accessory dorso-branchial lines 

 in Tremataspis and one in Bothriolepis. 



