rudimentary, 1-jointed and have a very short seta, and so they are in the seven remaining 
species belonging to the group of Spher. Leuckartu Sal. (comp. the systematic part). 
The Rostrum. It is always of good size and bluntly conical, or like a cylinder 
with dilated base. Its structure is very complex, and we will begin by studying its distal 
part, for the representation of which Choniostoma Hansenii (pl. X, fig. Ga and fig. 6b) will 
serve as type. In fig. 6a the cylinder is seen sideways and without the expanded part at 
its base. At the margin of the terminal face of the cylinder origimates a membrane which 
has the shape of a kind of border or very short inverted cone. In looking at it from the 
distal end (fig. 6b) we see that the membrane covers the whole terminal face, having the 
shape of a cup or perhaps rather of a flat funnel, as it leaves an oblong aperture at the 
bottom in the centre; this is the entrance of the mouth, beyond the margin of which the 
points of the mandibles are seen to proceed. In front of the mouth the membrane is divided 
in the middle by a deep incision; the opening thus produced is filled by an odd median plate, 
on each side of which is another plate which is partly covered by the membrane. In the 
illustration these parts are marked d. The membrane is downy at its edge (fig. 6b), and 
the whole inner surface of the funnel is covered with peculiar dots, which are smaller near 
the edge than towards the centre, and which probably represent tiny knots. Outside the 
membrane are seen a number of cylindrical hairs which are sometimes furcate at the apex 
(b). They are articulated to the distal edge of the cylinder at the base of the membranous 
border and, being longer than its height, proceed somewhat beyond its free margin. When 
— as in the present instance — the rostrum is cut off, it is easy enough to see that these 
hairs do not exist within the membrane, but only lean against it. In some species, e. g. in 
Spheronella curtipes (pl. X, fig. 2d), the membrane (viz. the free part of it) is considerably 
broader, in others narrower, than in Choniostoma Hansenii. The hairs in some species are 
much more numerous and much thicker than in others, and they often converge or diverge very 
irregularly, according to the position they happen to occupy; in a few species I was not able 
to discern them. My figures as a rule are too small to allow of drawing the membrane, but 
these hairs are drawn as well as it could be done. It must be observed that the shape of 
the mouth varies considerably in specimens of the same species; [ have found it more or less 
funnel- or cup-shaped, in accordance with the angle formed by the membranous border and 
the surrounding hairs against the terminal face. In the systematic part of the present work 
the free part of the membrane together with the hairs is called the mouth-border. 
The outer surface of the rostrum shows several harder chitinous lists, and when the 
rostrum is examined from its distal end, some harder parts are seen through the semi-dia- 
phanous membrane as circles, which are interrupted in front at the median line. In my 
opinion the distal part of the rostrum must be explained as being a highly modified Jabiwm, 
or rather hypopharyna, which forms a kind of sheath round the mandibles and stretches so 
far towards the front that its edges approach very near to each other, and that the above- 
mentioned median part marked d. must be considered as the /abrwm. However, I am not 
4° 
