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BULLETIN OE THE UNITED STATES FISH COMMISSION. 
organs no longer having this relation. The lower, originally left, toe is now next 
to the bottom when the animal is creeping, and will more often come in contact 
with it than will the right toe. Moreover, when a thread of mucus hangs from the 
toes and catches on some object on the bottom, it will more often be that from 
the lower (left) toe. 
So, perhaps as a consequence of this change of position and of relation to the 
environment, the right or upper toe begins to degenerate. The steps in degenera- 
tion are easily traceable and are shown in text-figure 1. In Diurella stylata Eyferth 
(6)'and I), brachyura Gosse (c) the toes are almost equal, but the left is a little longer. 
In Diurella porcellus Gosse (d) the difference is greater. In Diurella insignis Her- 
rick (e) and D. tenuior Gosse (/) the right toe is about half as long as the left. In 
Rattulus gracilis Tessin ( g ) it is about one-third the length of the left. The right 
toe now forms a small spine, which lias its tip bent toward the main or left toe, and 
lying against the latter. Rattulus lophoessus Gosse (h) shows a still farther step; 
R. elongatus Gosse (i), R. longiseta Schrank (./), R. scipio Gosse (A - ), R. earinatus 
Lamarck (J ) , R. viulticrinis Kellicott (to), and R. pus iUus Lauterborn (n) still farther 
ones in the reduction of the right toe and corresponding increase in the left one. 
In the species last named (i to ») the rudimentary right toe has usually been classed 
with the substyles; it can generally be recognized, however, by its form and posi- 
tion, as well as, at times, by the fact, shown in j, that one of the mucus reservoirs 
opens at its base. Finally, there are certain species, as Rattulus bicristatus Gosse 
(o), R. mucosus Stokes, and others, in which it is very difficult, or impossible, to 
distinguish between the rudimentary right toe and the substyles. 
It is probable that this degeneration of one of the toes is related primarily to the 
habit, so common in the Rattulidoe, of becoming suspended from foreign objects by 
a thread of mucus attached to the tip of the toe, and then revolving on the long axis. 
It is evident that a single, long rod is much better fitted to serve as a pivot than two 
toes side by side. These would impede the revolution by furnishing resistance to 
the water. 
The substyles are present all through the series. Their number varies; in most 
cases each of the two toes seems to have two, three, or four. In Rattulus bicristatus 
Gosse (text-figure 1, o) at least eight can be seen about the base of the main toe; 
among these the rudimentary right toe can hardly be distinguished from the others. 
Hand in hand with the reduction of the right toe goes a reduction of the mucus 
reservoir which is connected with it. The reduction of the mucus reservoir is not 
so extensive as that of the toe, and it never, completely disappears. Indeed, in some 
cases where the toes are very unequal, the two reservoirs remain of the same size. 
This is true in Rattulus stylatus Gosse (fig. 92, pi. x.) Unequal reservoirs are shown 
in text-figure 1 , j and o. 
Apparently, in some species at least, the toes are of the full length when the 
animal is hatched from the egg, while the body is much smaller than it later becomes. 
Thus in young specimens the toe is much longer in proportion to the body than 
in adults. This is well shown by comparing figs. 18 (pi. ii), 51 (pi. v), 90 (pi. x), 
representing young specimens respectively of Diurella insignis Herrick, Rattulus 
scipio Gosse, and R. mucosus Stokes, with the other figures representing adult 
specimens of these species. This is a point worthy of special note, as it may easily 
lead to error in specific determinations. 
