230 FAUNISTIC NOTES AT PLYMOUTH DURING 1898-4. 
and an incessant sinking to the bottom of surface larvee whose float- 
ing period has come to an end. Were our work completed, our 
calendar of the floating fauna would largely coincide with, or bear 
a definite relation to, our calendar of the breeding seasons of bottom- 
living animals; but the insufficiency of present records, combined 
with their obvious want of mutual correspondence, shows what a 
large amount of observation and work remains to be done before the 
desired pitch of correspondence is attained. 
The periodic character of the floating fauna is also manifested in 
other ways, and especially in certain seasonal changes which the 
gradual rise or fall of temperature during the year superinduces. 
‘These changesare due to actual immigrations of forms whose homes for 
the most part are in other regions, but which arrive at particular 
points upon our coasts when the temperature and other physical 
conditions admit of, and conduce to, their migration. The periodical 
changes which were first dealt with here were those of larval or 
metamorphic forms (e.g. most medusze), whose derivation from the 
bottom fauna, and, in most cases, whose eventual return to it give 
rise to an incessant vertical interchange of material; but the forms 
which are especially concerned in these seasonal changes are for the 
most part creatures whose entire existence is pelagic (e.g. Siphono- 
phores, Trachomedusx, Copepods, Sagitta, Salpa), and which exhibit 
a merely horizontal translation from place to place according to the 
stress of physical conditions. 
The situation of Plymouth at the head of a landlocked bay and 
at the western end of the English Channel renders the study of its 
floating fauna both difficult and interesting. This fauna is in fact 
not one fauna, but three faunas,—that of the harbour itself, which 
may be called the indigenous element ; that of the adjacent coasts and 
of mid-Channel, which may be termed the tidal or Channel element ; 
and that of the Atlantic, or the oceanic element. It is only by 
taking special precautions that one can isolate the first of these con- 
stituents from the other two, viz. by tow-netting within the Break- 
water at low tide, when the wind has been light or northerly. At 
high water the Sound is naturally invaded by foreign forms carried 
inwards by the flood tide, and this may include eddies from the 
Channel tide properly so called; while protracted southerly or 
westerly winds inevitably produce an incursion of pelagic forms from 
the Channel or the ocean. <A special study of the floating fauna 
would therefore take all these points into practical consideration, 
and I mention them here in order to warn those who may use these 
brief notes of the different factors of which the fauna at Plymouth 
consists, and of the impossibility, after the irregular experience of a 
few years only, of producing a calendar which shall in all points 
