152 WwW. B. SCOTT. 
ridge, between the mouth and olfactory pit, which seen in 
longitudinal vertical section, has the shape of a right-angled 
triangle, the hypotenuse of which ibounded by the olfactory 
pit. This stage is found in embryo of about the eighteenth 
day. Invery small larve the lower edge of the ridge (7. e. the 
edge next the mouth) has begun to grow very rapidly, and is 
curved downwards and backwards. The mouth has still a com- 
pletely ventral situation; the correction of the cranial flexure 
brings it further and further forwards, and at the end of this 
the upper lip, whieh is now still more lengthened, turns 
about an angle of nearly 180°, so that the edge of the lip, 
which formerly pointed directly backwards, now points directly 
forwards. 
This gives us the characteristic terminal mouth of the Cyclos- 
tomata, the formation of which brings the olfactory pit to the 
upper side of the head. According to Max Schultze’s results, 
the mouth is fitted for its sucking function in almost the very 
smallest larvz. 
This peculiar mouth would, therefore, appear to be one of the 
first deviations from the normal character which developed itself 
- in this group, and the change which the mouth undergoes is 
followed by, and I think the connection is a causal one, many 
other alterations, e.g. the situation and fusion of the olfactory 
organs. The change of the mouth into a sucking apparatus 
makes necessary an alteration of the mechanism of breathing. 
The water of respiration can no longer stream through the mouth 
to the gills and out of the gill-slits, but must stream in and out 
of the gill-slits; and this makes necessary a change in the 
musclés and skeleton of the gill apparatus. These changes 
cannot be discussed here, we can only indicate the general im- 
portance of the formation of the mouth. In addition to the 
modifications already mentioned, there must be remembered the 
new formations, the supporting cartilages for the sucking-disc, 
none of which are present until after the metamorphosis of the 
larva into the sexual animal, as well as changes in the course of 
the cranial nerves, which are easy to follow in the course of 
development. In short, I jind im the change of form of the 
mouth a key to the solution of the problem of the head and its 
organs in the Cyclostomata. 
Urino-genital system.—My observations upon the excretory 
system are somewhat more complete than those of W. Miller 
(‘Jen. Zeitschr.,’ B. ix), but they confirm all his results. The 
“segmental duct ” (I use this term of Balfour’s to translate the 
“‘ Kopfnierengang”) is formed as a solid cord in the lateral part 
of the mesoblast, which is not taken up in the formation of the 
protovertebre. This solid cord appears on about the fourteenth 
