ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 315 



the superficial cells of the epithelium again assume the mucin-secreting character as in 

 the mouth. The epithelium gradually decreases in height from 72 m up to the 

 boundary, where it is only 40-48 m high. The transition is quite sudden. It does not 

 occur at the same level in all the folds — otherwise a straight line would accurately 

 divide the two types of epithelium. The single-layered epithelium of the abdominal 

 gut is at first without the striated border, nor does it possess any gland cells. It very 

 gradually increases in height, and acquires first the striated border and then its charac- 

 teristic unicellular glands. 



Submucosa. — Dense and fibrous in the branchial gut, but immediately in front of 

 the constrictor cardige it is largely replaced by a thick layer of fatty areloar tissue. In 

 the region of the constrictor, however, the submucosa is again, and even more, dense 

 and fibrous, and becomes increasingly so as it passes backwards. Behind the constrictor 

 {i.e. at the epithelial change) it gradually diminishes, and passes without a break into 

 the stratum compactum of the abdominal gut. There is no doubt that the latter layer 

 is only another form of the same tissue as the submucosa of the pharyngeal gut, 

 although its staining reactions are slightly different. The curious submucous blood 

 sinus associated with the stratum compactum may be traced as far forwards as the 

 epithelial change. 



Musculature. — ^The unstriated musculature gradually increases as we trace the 

 branchial gut backwards, and it is a very conspicuous layer in the fatty region im- 

 mediately in front of the constrictor cardiae. It diminishes somewhat at the anterior 

 end of the constrictor, but increases again as it passes backwards, decreasing once more 

 at the epithelial change, finally becoming directly continuous with the weak unstriated 

 musculature of the abdominal gut. The new layer, therefore, in the latter gut, which 

 is not represented in the branchial gut, is the thick zone of fatty lymphatic tissue 

 between the stratum compactum and the unstriated musculature. 



The entrance of the bile duct marks no change in the character of the gut, and 

 hence 1 am unable to agree with Maas that there is a representative of the stomach 

 in Myxine. The epithelium, stratum compactum, and the lymphoid tissue are the same 

 both in front of and behind the aperture of the bile duct, although the typical abdominal 

 gut extends only a very short distance in front of this point. 



The passage of the mid- or abdominal gut into the hind-gut or cloaca is also best 

 observed in serial longitudinal sections. We thus find : — 



Epithelium. — Insensibly diminishes from 120 m, losing its gland cells and its 

 striated border, until at the boundary it is only 32-56 m high. Maas figures the 

 striated border and the gland cells right up to the change, a condition not found in 

 any of my preparations. The single-layered epithelium of the abdominal gut meets 

 the many-layered epithelium of the cloaca at a bevel joint, dissimilar to the junction 

 of the branchial and abdominal guts, and in such a way as to suggest that the abdominal 

 epithelium represents the gradually heightened superficial layer of cells of the cloacal 

 epithelium. The latter increases slowly in height up to 80 m, and for some distance 



