508 PEOr. T. W. BRIDGE ON THE 



any rate in the region of the air-bladder. At the commencement 

 of the tail only the ventral third of the radial elements serve for 

 the origin of the flexor muscles of ihe anal fin-rays, the dorsal 

 two-thirds, and the short haemal spines with which the former 

 iiiterdigitate, being quite free from muscular attachment. "With 

 the gradual shortening of the radial elements towards the middle 

 ]jortion of the tail, where the air-bladder terminates, nearly the 

 whole length of each radial element becomes invaded by the origin 

 of the flexor muscles. The area over which the various radial 

 elements are wholly devoid of muscular attachment is coextensive 

 with that of the inner wall of each of the two closely-related 

 caudal ca^ca. The series of radial elements and haemal spines are 

 connected together by a thin, but tough, sheet of fibrous tissue, 

 which extends Irom one to the other and fills up the intervals 

 between them, and, with the caudal skeletal elements above 

 mentioned, form the only separation between the caudal caeca of 

 o])posite sides of the tail. It may be added, that the series 

 of ventral diverticula occupy the intervals between the successive 

 pairs of radial elements. 



The dorsal and outer walls of each caudal csecum are of 

 moderate thickness, as also are the external Avails of the ventral 

 diverticula, while tlie filiform cfeca have much thinner walls. In 

 each case, nevertheless, a relatively thick outer fibrous stratum 

 or tunica externa, and an extremely thin lining or tunica interna, 

 consisting of an internal epithelial stratum, supported externally 

 by a thin layer of connective tissue, can readily be recognized. 

 The inner wall of each caudal caecum, including also that of each 

 of its primary ventral diverticula, on the conti'ary, is of extreme 

 tenuity, consisting only of the tunica interna, and, moreover, is 

 closely adherent to the outer surfaces of the radial elements and 

 hsemal spines, and also to the fibrous sheet which stretches between 

 them. In fact, in the intervals between the^e skeletal elements, 

 the median fibrous sheet and the attenuated inner walls of the 

 caudal caeca are all that separate the cavities of the cseca of 

 opposite sides of the tail ; and so thin are the inner wails of 

 these that, when the cavity of either of them is exposed by the 

 removal of its outer wall, the various skeletal elements {Ji.s., r.e.) 

 appear as if completely bare of any investing tissue and to 

 project freely into the lumen of the bladder. 



In addition to the investment of the cuter surfaces of the 

 caudal skeletal elements by the tunica interna of the inner wall, 



