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Second inethod uj
explanation.
Polycopidae.
Sarsiellidae.
Cypridinidae.
Asleropidae.
one- to four-jointed part of this limb seems to be homologous with the branch that was
termed the exopodite in the preceding families. Cf. ligs. IV: 7, 8.
According to the second of the two methods of explanation mentioned above the fifth
limb in the different families is in my opinion to be explained as follows:
The vibratory plate corresponds to the exopodite. The greatest difficulty in carrying
out this homologization arises when we have to define the boundary between the protopodite
and the endopodite. In this but rather little value can be attached to the position of the
vibratory plate; it is very far from improbable that this organ has been subject to not
inconsiderable alterations in position. The difficulty in fixing this boundary was really so
great that it seemed to me impossible to reach anv definite result; there have always seemed
to be different possibilities present.
Family Polycopidae: The protopodite either has the same extension as it has according
to the first method of explanation, i. e. it is more or less distinctly three-jointed, with a
pro-coxale, a coxale and a basale, or else it is formed only by the two joints on wliich the vibratory
plate is fixed. In the first case the nearest joint distally to the vibratory plate would correspond
to the basale and the outer of the two distal processes would correspond to the endopodite,
the inner being an accessory appendage, an endite on the basale, cf. fig. VI; in the second
case the first-mentioned joint woiüd correspond to the first endopodite joint and one of the
two distal processes is to be regarded as the end joint of the endopodite, the other as an
accessory appendage to the first endopodite joint. In the former case the vibratory plate,
the exopodite, has been displaced proximally, but not in the second case.
Family Sarsiellidae: As is seen above, this limb is almost entirely without any division
into joints. The large outer distal process is to be regarded as an endopodite, the small inner
distal lobe presumably as an endite either on the basale (cf. the accompanying figure VI)
or on the endopodite.
Family Cypridinidae: The protopodite either has the same extension as according to
the first method of explanation or else it is represented only by the joint (or the two joints)
on which the vibratory plate is fixed. In the former case the joint nearest to the vibratory
plate distally is to be homologized with the basale, and the following joints correspond to the
endopodite; with this homologization the part that was denoted, according to the first method
of explanation, as the first and second exopodite joints would correspond to the first and second
joints of the endopodite; cf. the accompanying figure VI. In the latter case the joint situated
distally nearest to the vibratory plate is to be considered as the first endopodite joint; the
endopodite should have five joints in the accompanying figure. In the former case the exopodite
has been displaced proximally.
Family Asteropidae: The comb seems to have been formed from the protopodite + the
two proximal endopodite joints or from the protopodite and the three proximal endopodite
joints. The long bristle (or the two long bristles) with the short bristles close to it
(or them) on the lateral side of the comb appears, according to this interpretation, to
belong to the two distal joints of the endopodite or to one of these two joints; cf. the
accompanying figure VI.
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