122 GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS. 



more, the two kinds of organs, so that each kind assembles in particular areas 

 and is supplied with distinct nerves arising from distinct brain tracts. We shall 

 here refer to the common anlagen of both sets of organs as coxal and vagal sense 

 organs. 



There is a sharp distinction morphologically between the anlagen of the 

 thoracic organs and those of the vagal region. The thoracic anlagen are always 

 directed forward and outward and are located well on the sides of the thorax. 

 The vagal anlagen are always crowded close to the median line and are directed 

 backward, approximately parallel with the nerve cord. (Fig. 89.) The location 

 and direction of growth of these organs is determined by that of the appendages to 

 which they belong and is prophetic of their condition in vertebrates. 



When the oral appendages were transferred to the haemal surface (see Chapter 

 XV), it is probable that the anlagen of the coxal organs were drawn forward and 

 outward into a narrow band, each one giving rise to a row, or linear series 

 of taste organs, the general course or direction of the organs, and the accompany- 

 ing nerves and ganglia, indicating the path of migration of the corresponding 

 appendage. 



The vagal appendages of the arachnids are always carried backward, relative 

 to the other parts of the same segments, as shown by the invariable direction of 

 their nerves and ganglia. The conditions that controlled their movements have 

 no doubt continued to direct the line of growth of the vagal group of anlagen in 

 the embryos of their vertebrate -descendants. 



There is nothing to indicate what conditions determined the backward growth 

 of the immense longitudinal cutaneous nerve, which in Limulus arises from the 

 first post-oral neuromere. (Figs. 70, 89, l.n.) 



The embryological history of the lateral line organs in primitive vertebrates 

 bears out this interpretation. We may recognize there two principal groups of 

 organs, one lying in front of the auditory organ and belonging to the oral arches, 

 the other lying behind the auditory organ and belonging to the branchial region 

 and trunk. The former repiesent the coxal organs of the arachnid thorax, the 

 latter, the organs of the vagal appendages. These two groups of organs grow in 

 the same general direction in the vertebrates that they do in the arachnids, but 

 they have extended very much farther in the former. 



In the vertebrates the several pairs of anlagen tend to run together, and it 

 is not clear just how many there are in either region, or which ones of those in the 

 arachnids they represent. 



The second, third, fourth, and to a less degree, the fifth pairs of coxal anlagen 

 en the arachnids are probably in part retained in the vertebrates, forming the 

 rudiments of lines of canal organs for their corresponding appendages, which 

 have themselves furnished the basis of the premaxillary, maxillary, mandibular 

 and hyoid arches. One or more groups of vagal sense organs gave rise to the 

 lateral lines of the branchial region and the trunk. 



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