ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 335 



sind theils cubisch, 0,008 im Durchmesser, theils cylindrisch, 0,012 hoch, 0,008 breit, 

 sammtlich mit Kernen von durchschnittlich 0,006 und 1-2 Kernkcirperchen und sehr 

 zartem, feinkornigem Protoplasmakorper versehen. Das Epithel umschliesst eine scharf 

 begrenzte rait klarer, farbloser Fliissigkeit gefiillte Hohle." This description, though 

 brief, admits of no doubt as to Muller having seen the thyroid of Myxine. The same 

 writer a year later* describes the thyroid of the adult lamprey as extending below the 

 longitudinal " tongue " muscle from the second to the fourth pair of gill sacs. 



The development of the Myxinoid thyroid has been studied by Stockard in 

 Bdellostoma (5S). He finds that it arises as a median, unpaired " downpushing from 

 the ventral floor of the pharynx throughout the entire gill area." This forms a long 

 trough with a restricted lumen, which subsequently becomes separated from the gut, 

 and breaks up to form the isolated vesicles of the adult. " The chief point of interest 

 shown by the trough-like thyroid anlage is the very extensive gut area from which it 

 is derived ; in no other vertebrate does the thyroid evagination from the pharynx run 

 through so relatively long an area " (p. 95). " It will now be clearly seen that although 

 ihe adult condition of the thyroid in Myxinoids and Teleosts are readily comparable, 

 the developmental processes through which the ends are reached seem widely different " 

 (pp. 97-8). Judging from the recent work of Ferguson and Gudernatsch, the thyroid 

 is a remarkably uniform structure throughout the Chordate series. 



J. Cloaca (Fig. 5). 



The cloacal aperture is an elongated longitudinal slit compressed from side to side, 

 9 mm. long in a 36-cm. Hag, and 4|- cm. from the extremity of the tail. The anatomy 

 of the cloacal region was first fully described by R. H. Burne (12). If an incision be 

 made from the anterior extremity of the cloacal aperture and continued a short distance 

 along the rectum, and the walls pinned out, it will be noted that the anus (an.) opens 

 into the cloaca (cl.) anteriorly and ventrally by a larger puckered aperture. The roof 

 of the rectum [an.') is continued backwards across the cloaca at the sides, but not at 

 the middle line, and thus the cloaca is divided imperfectly into a dorsal and a ventral 

 chamber. Both chambers bear folds. In the one case they are continued backwards 

 from the margin of the genital pore, and in the other they are the direct continuations 

 of the rectal folds. The ventral chamber receives the anus, as already mentioned. 

 Into the dorsal chamber there opens in front, and of course dorsal to the anus, the 

 single, conspicuous, round abdominal pore, or porus genitalis (p.g.), surrounded by a 

 thick fibrous band {p.g.'). This pore was first described by A. M. C. Ddmeril in 1807, 

 in Myxine, and afterwards by J. Mijller. From the porus genitalis there passes back- 

 wards right to the posterior end of the cloaca a narrow but obvious median dorsal ridge. 

 Somewhat in front of the middle of this ridge there is a thickening, the urinary papilla 

 {ur.pp.), and an examination of this thickening with a lens reveals two small 



* Jena. Zeits., Bd. vii., 1872. 



