56 DE. W. 0. EIDEWOOD ON THE 



extremity is seen not to partake of the general lateral curvature, 

 or only in a minor degree. The caudal fin is chiefly concerned 

 with obtaining a purchase upon the water, so as to constitute a 

 relatively fixed point, about which the rest of the body can be 

 moved by appropriate muscular contraction. Flexibility is thus 

 not required in the caudal fin itself, but is of great advantage in 

 the part of the body immediately preceding. The need for this 

 extreme flexibility ceases in front o£ the cloaca, for here the body 

 is largely occupied by the alimentary and other viscera, and 

 constitutes, with the head, the most important part of the body, 

 compared with which the post-cloacal part is merely a sub- 

 servient appendage. And, further, since the body is thicker in 

 the trunk region, the proportion existing between the length and 

 the breadth of a segment is much less than in the tail, and con- 

 sequently one vertebra to eacli myomere gives the necessary 

 amount of flexibility. 



There are not, in Sharks, synovial articulations between the 

 centra as in Snakes, where bhe flexibility of the vertebral column 

 is considerable; neither are there zygapophysial articulations 

 between the neural arches as in most Vertebrates. The only 

 movements possible are those due to the slight yielding power 

 of the fibrous tissue around the margins of the centra, and 

 between the various cartilages of the neural arches. To double 

 the yielding power of this separating fibrous tissue would be to 

 weaken the connection between the several vertebrae, and to 

 iutroduce the possibility of lateral displacement ; but by doubling 

 the number of vertebrge in any region, twice the amount of 

 fibrous tissue is introduced, without the above-mentioned dis- 

 advantage. 



This response by the skeletogenous tissue to the requirements 

 of flexibility of the particular part of the body, is possibly 

 referred to in the following sentence from Gradow (4. p. 192) : — 

 " It is obvious that the chondrified chordal sheath is affected by 

 the ' centra of motion,' which establish themselves according to 

 the way in which the fish ' wriggles '." 



That the vertebrae must be integral multiples of the segments 

 of the body is evident from the relations which exist between 

 the muscles and the vertebrge. Although a secondary feature 

 (Balfour, 1. p. 453, and Gadow, 4. p. 192), it is a fact, that in 

 the development of Elasmobranch fishes the chondi-ified sheath 

 of the notochord is uniform and unsegmented at a time when 



