"POEUS GENITALIS " IK THE MYXiKlDiE. 491 



while the ridges themselves are formed by a backward extension of 

 the interlobular connective tissue. In other words, the grooves 

 between the upper series of cloacal ridges have at their anterior 

 ends the structure of the lateral slime-glands, and are prolonged 

 as glandular conical pockets for some distance into the lateral 

 and ventral walls of the pore. I have gone at some length into 

 the structure of this gland, as I believe that hitherto the features 

 peculiar to the lateral slime-glands have been found there, and 

 nowhere else*. 



The ureters (fig. 1, ur.) run one on either side of the dorsal 

 mid-line, outside the peritoneum ; towards the posterior end of 

 the body-cavity they approach each other, then run backward 

 close side by side upon the dorsal wall of the cloaca, and finally 

 open into it by a pair of slit-like openings upon the urinary 

 papilla (fig. 1, ur.o.). 



In addition to Myxine glutinosa I have been able to make a 

 dissection of Bdellostoma cirrhatum, and find that in this genus 

 the " porus genitalis " differs in several points from that of Myxine. 

 In the first place the pore (fig. 2, p.g.) is very large, ex- 

 tending for 6 mm. on either side of the middle line ; it is also 

 situated considerably farther forward than in Myxine, lying 

 14 mm. in front of the anus instead of directly above it. The 

 forward position of the pore is accompanied by a corresponding 

 forward extension of the cloacal chamber upon the dorsal surface 

 of the rectum, and by the obliteration of the unpaired portion of 

 the body-cavity that in Myxine lies between the posterior edge of 

 the dorsal mesentery and the pore. Thus we are practically dealing 

 with a pair of genital pores, each 6 mm. broad, lying close side by 

 side upon the dorsal surface of the rectum and separated one 

 from the other by the edge of the mesentery. At first sight, one 

 is sorely tempted to regard this condition as intermediate be- 

 tween the paired lateral pores of the Lamprey and the single 

 median pore of Myxine — a step in the formation of a dorsal pore 

 by the coalescence of a pair of lateral pores ; but this is prob- 

 ably not the case. For when we notice that the body-cavity 

 of Bdellostoma does not extend backward beyond the position 

 of the edge of the mesentery in Myxine, and that the end of 

 the rectum covered in Myxine by peritoneum is in Bdellostoma 

 clothed by a highly specialized extension of the cloacal epi- 



* Bloomfield, "The Thread-cells and Epidermis of Myxine,'' (^xxaxi. Journ- 

 Micr. Sci. xxii., 1882, p. 355. 



