168 '
G. H. PARKER
,
1
muscles contract in sequence
from the anterior to the posterior
end instead of the reverse and the longitudinal muscles follow
the same sequence; otherwise they act as they do in the direct
type.“
It is evident from this brief discussion of the nature of the
.
waves in the retrograde type that the theory developed in connec—
'
tion with the direct type applies perfectly to this second type.
'
It remains still to point out that what I have called the arhyth—
.
mic form of pedal locomotion, a form well exemplied in Ilyan—
assa, may be explained on the same general basis as that which
_
,
has just been given for the two types of arhyhmic locomotion. If ‘
the foot of such a snail as Ilyanassei be thought of as composedof
,_
a multitude of small areas, each one of which can be ‘lifted from
'
,_
the substrate, moved forward, and set down again separately,
_
and that this action takes place irregularly and Without reference
'
to any sequence, it can easily be seen how the animal could move
'
forward but
without the formation of pedal waves. lt is my be—
.
_
lief_ that this is the condition in the foot of the arhythmic gastro—
'
pods,
but becaUse of the Small size of Ilyana‘ssa, I have not been
‘
«
able to Subject this
to experimental test.
.
'
B-Gf01“€ ClOSig this paper,
I wish to add a word concerning the ,7'
_
Very “Temarkable
method of locomotion observed by Carlson (’Oô)
in' Helix
_Th‘e movement carried out. by this
snail
ÎS appr0priately deScribed as a. gallop, both from its rate and
.
Ï "
COnguration.‘ The sail
provocation lifts the head and
»
_
.
Pf0jeCtS
ÎOI‘WäJTd,‘ ad eventually brings‘it to. the
_
thus
mitieti%.a giant Wave
length
°-f
bépresentataidnce;— Carl
1000m@t10and1suspectthatthœsnælalsoassessesthetypmal
Snä“lWhemgaopbee€X-‘Mlledfmmbel@Wthepedalvvewes
W©davebænseenmemtlnm0®munctwnwmhtheoy