556 DAVIDSON BLACK 
together with the relatively slight development of gustatory 
and vestibular organs have necessitated a corresponding read- 
justment of their life habits and a consequent limitation of their 
range and simplification of their motor reactions. In order to 
exist at all these animals must seek and occupy a favorable 
environment where adequate protection may be had and where 
food may be obtained successfully despite the limitation of their 
sensory equipment. 
Success under these circumstances means specialization and 
the degree of success with which myxinoids have attained this 
end is admirably exemplified both by the survival of the type 
under modern conditions and by the highly specialized motor 
nuclear pattern already described. 
That the motor nuclear pattern is specialized in myxinoids 
is but another way of stating that the neurones in question 
occupy a position most favourable for the reception of reflex 
impulses from the dominant afferent nuclei of these animals. 
Among elasmobranchs the vestibular, visual and lateral line 
sense organs are all functionally well developed and with the 
aid of gustatory and tactile sensibility provide ample means 
whereby these forms may receive information of a most varied 
character concerning their environment. Within the brain 
stem in these forms the terminal sensory nuclei are developed in 
correspondence with this receptor equipment so that, in con- 
trast to myxinoids, no one constituent of either somatic or 
visceral areas can be said to dominate the anatomical arrange- 
ment of the medulla. On this account the organization of the 
afferent divisions of the nervous system within the medulla is 
of a more generalized type among elasmobranchs than in 
myxinoids. 
One of the chief differences between the motor nuclear pat- 
tern of cyclostomes and that of elasmobranchs les in the asso- 
ciation of the motor VII elements with those of the vagus and 
glossopharyngeus to form the caudal viscero-motor column 
among all members of the latter group. This arrangement has 
undoubtedly taken place under the influence of the elements 
within the well developed communis area and has resulted in 
