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 Hne arises from it and extends hjemally 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. 



premaxillas. 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. 



