LOCOMOTION. 99 
HABITS OF LOCOMOTION. 
The methods of locomotion may be deduced with some safety from a study of the ap- 
pendages, and, as has repeatedly been pointed out, all trilobites could probably swim by their 
use. This swimming was evidently done with the head directed forward, and could prob- 
ably be accomplished indifferently well with either the dorsal (gastronectic, Dollo) or the 
ventral (notonectic) side up. If food were sought on the bottom by means of sight, the animal 
would probably swim dorsal side up, for by canting from side to side it could see the bottom 
just as easily as though it were ventral side up, and at the same time it would be in position 
to drop quickly on the prey. In collecting food at the surface, it might swim ventral side up. 
All trilobites could probably crawl by the use of the appendages, and, as has already 
been pointed out, there are great differences in the adjustment of the appendages to different 
methods of crawling. Some crawled on their "toes," some by means of the entire endopo- 
dites, and some apparently used the coxopodites to push themselves along. That the normal 
direction of crawling was forward is indicated by the position of the eyes and sensory anten- 
nules. There is no evidence that their mechanism was irreversible, however, and the position 
of the mouth and the shape of the hypostoma indicate that they usually backed into feeding 
position. The caudal rami of Neolenus were evidently sensory, and the animal was pre- 
pared to go in either direction. 
The use of the pygidium as a swimming organ, suggested by Spencer (1903, p. 492) on 
theoretical grounds, developed by Staff and Reck (1911, p. 141) from a mechanical stand- 
point, and elaborated in the present paper by evidence from the ontogeny, phylogeny, and 
musculature, provided the animal with a swifter means of locomotion. By a sudden flap 
of this large fin, a backward darting motion could be obtained, which would be invaluable 
as a means of escape from enemies. Staff and Reck seem to think that in this movement 
the two shields were clapped together, and that the animal was projected along with the hinge- 
like thorax forward. This might be a very plausible explanation in the case of the bivalve- 
like Agnostidse, and it is one I had suggested tentatively for that family before I read Staff 
and Reek's paper. In the case of the large trilobites with more segments, however, it would 
be more natural to think of a mode of progression in which there was an undulatory move- 
ment of the body and the pygidium, up-and-down strokes being produced by alternately 
contracting the dorsal and ventral muscles. Bending the pygidium down would tend to pull 
the animal backward, while bringing it back into position would push it forward. It fol- 
lows, therefore, that one of these movements must have been accomplished very quickly, the 
other slowly. If the muscle scars have been interpreted properly, the ventral muscles were 
probably the more powerful, an indication that the animal swam backward, using the cephalon 
and antennules as rudders. 
The chief objection to the theory of swimming by clapping the valves together is that 
where the thorax consists of several segments it no longer acts like the hinge of a bivalve, 
and a sudden downward flap of the pygidium would impart a rotary motion to the animal. 
Take, for example, such nearly spherical animals as the Illa;nidje, and it will readily be seen 
that there is nothing to give direction to the motion if the pygidium be brought suddenly 
against the lower surface of the cephalon. A lobster, it is true, progresses very well by 
this method, but it depends upon its great claws and long antenna; to direct its motions. 
The whole shape of the trilobite is of course awkward for a rapidly swimming animal. It 
could keep afloat with the minimum of effort and paddle itself about Avith ease, but it was 
not built on the correct lines for speed. 
