PLANKTONIC STUDIES. 605 
hand we know of no neritic tunicates, no other forms of swimming 
mantled animals which are found only on the coasts, except the omni- 
present ascidian larva. 
Lately Chun has established the interesting fact that the planktonic 
tunicates occur in numbers not only at the surface and in slight depths, 
but also during the summer extend down into greater depths (15, pp. 
32, 42). He discovered further in the Mediterranean new Copelata, 
which are only zonary or bathybic, never coming to the surface and 
characterized by peculiar organization as well as difference in size 
(Megalocereus abyssorum, 3 centimeters long, 15, p. 40). 
The small, delicate Copelata and Doliola, from their small size, are 
naturally more difficult to see than the large luminous Salpe and 
Pyrosoma. Whoever has carefully examined great quantities of oceanic 
plankton can readily testify that the former also occur almost every- 
where and oceasionally take an important part in the constitution of 
the mixed plankton. Among the Salp@ there are for example the 
smaller species which form extensive swimming shoals. From the 
three-year observations of Schmidtlein it is learned that the salpas 
belong to the perennial plankton and are numerous throughout the 
whole year (19, p. 123). 
K.—VERTEBRATES OF THE PLANKTON. 
The vertebrates of the sea are in their mature condition for the most 
part too large and have too powerful voluntary movements to be 
reckoned in the true plankton in Hensen’s sense, as “animals carried 
involuntarily with the water.” The sea fishes, as well as the aquatic 
birds and mammals of the sea, overcome more or less easily the impetus 
of the currents, and thereby prove their independence by voluntary 
movements, which is not commonly the case with the floating inver- 
tebrate animals of the plankton. Meanwhile I have already shown 
above that this limitation of the plankton against the nekton is very 
arbitrary and at any moment may be changed in favor of the latter 
through diminution of the strength of the current. For the chief point 
of Hensen’s plankton investigation, for the question of the “cycle of 
matter in the sea,” the vertebrates are of greatest importance, since 
they, the largest of the rapacious animals of the sea, daily consume the 
greatest quantity of plankton, no matter whether directly or indirectly. 
A single sea fish of medium size may daily consume hundreds of 
pteropods and thousands of crustacea, and in case of the giant cetaceans 
this quantity may be increased ten or a hundred fold. In a@ compre- 
hensive consideration of the plankton conditions, and particularly in 
its physiological, cecological, and chorological discussion, a thorough 
investigation of the vertebrates swimming in the sea, the marine fishes, 
the aquatic birds, seals, and cetaceans, is not to be undertaken. We 
can then turn from it here, since it has no further reiation to the pur- 
pose of this plankton study. We can here in Hensen’s sense (9, p. 1) 
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