494 ON THE " PORUS GENITALIS " IN THE MYXINID^. 



direct relation that exists between the quantity of slime secreted 

 and the size of the pore, it looks as if it might answer somewhat 

 the same purpose as the wax secreted in our own external auditory 

 meatus, by preventing the intrusion of foreign matter, animate or 

 inanimate, into the body-cavity, a danger to which the possessor 

 of two contiguous pores, each 6 mm. broad, would seem to be 

 especially exposed. 



It will have been observed that the main diiFerence between 

 the Lampreys and Myxinoids lies in the absence in the latter 

 of the direct communication between the " porus genitalis " and 

 the uro-genital sinus that forms so characteristic a feature in the 

 former. The apparent absence of the uro-genital sinus in the 

 Myxinoids is, I am aware, generally explained by regarding 

 the upper part of the cloacal chamber as its representative ; but 

 the following features seem to indicate that its absence is not 

 apparent, but real. Although there seem to have been some 

 contradictory statements made * in regard to the development 

 of the cloaca in the Lamprey, yet the following account, taken 

 from a recent detailed paper on the transformation of Ammocoete 

 to Lamprey t, may probably be considered as fairly representing 

 the truth : — The segmental ducts Spen originally into the posterior 

 region of the gut, and then, during the passage of the Ammocoete 

 into the Lamprey, their posterior ends fuse to form a common 

 chamber into which, as it gradually separates from the gut and 

 acquires an opening into an integumentary cloacal pit, the 

 hitherto blind posterior prolongations of the body-cavity open, 

 forming the " genital pores." So that in the adult we can 

 sharply distinguish an uro-genital sinus, formed by the fusion of 

 the segmental ducts, from an integumentary cloacal pit into 

 which it opens. Now in the adult Myxinoid the ureters do not 

 imperceptibly pass into the cloacal chamber, as they do into the 

 uro-genital sinus of the Lamprey, but open upon a raised papilla ; 

 upon the margin of the ureteric opening, the epithelium changes 

 its character — inside it is similar to that lining the rest of the 

 ureter, outside it is epidermic. The whole cloacal chamber, both 

 in Myxine and Bdellostoma, is lined by epidermis. Tor these 

 reasons I am inclined to consider that the uro-genital sinus of the 



* Ayers, " Untersuchungen iiber Pori abdominales," Morph. Jahrb. x. (1885) 

 p. 346. 



t Bujor, " M^tamorphoBe de Y Ammocxtes hranchialis en Peti'om^zon Flaneri," 

 Eev. Biol. JSord France, iii. (1891) p. 484. 



