THE MARSIPOBRANCHII. 139 
alluded to, differences far greater and more remarkable 
than those between the lip supports of the Lamprey and 
Hag; and I am therefore constrained to regard the series of 
slender cartilages which in the Myxinoids support the lip 
(lb. i—iv fig. 2) as together equivalent to the Lamprey’s 
annulus, and (with Joh. Muller) as comparable to the labials 
of Elasmobranchs, regarding the differences between the 
two types as either due to divergence in functional modi- 
fieation or to degeneration upon the reduction of the 
sucking mouth.* If this be conceded, there is demon- 
strated a very marked structural unity between the 
Petromyzontide and Myxinide, in respect to which they 
hold together as clearly as, in the absence of paired fins at 
all periods of life,t and in other well known features, they 
stand apart from the remaining vertebrata. Their inter- 
relationships admit of a ready comparison with those of the 
existing Monotremes, except that in the latter there is no 
counterpart for the Bdellostoma type. 
VII. That the general organization of the Marsipo- 
branchs is a simplification of that of the higher vertebrata 
has long been recognized. The position of the heart, the 
structure of the kidney, the alternation of the roots and 
distribution of the spinal nerves (Petromyzon), are con- 
spicuous characters concerning which these animals, as 
compared with the higher gnathostomata, are in a per- 
sistently embryonic condition. While this is so, the study 
of the hypophysis shows them to be modified in a direc- 
tion altogether away from the other vertebrata. Despite 
the deal that has been written upon this organ, the signi- 
* Parker’s failure to discover a complex origin for the Lamprey’s annulus 
(Phil. Trans., 1883, p. 422) strengthens the latter view. 
f Dohrn’s alleged discovery (Naples Mittheilg, Bd. VI, pp. 406 et seq) of a 
pelvic-fin rudiment in Petromyzon appears to me to rest upon a wholly 
insufficient foundation. 
