PLANKTONIC STUDIES. 605- 



liaiid 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 tliat the planktonic 

 tunicates occur in luimbers 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 iMediterrauean new CopeJata, 

 wl'iich are only /onary or bathybic, never coming to the surface and 

 characterized by peculiar organization as well as difference in size 

 [Megalocerens abyssorim, 3 centimeters long, 15, p. 40). 



The small, delicate Copelata and Dolioln, from their small size, are 

 naturally more difficult to see than the large luminous 8alp(c and 

 Pijrosoma. Whoever lias carefully examined great quantities of oceanic 

 plankton can readily testify that the former also occur almost every- 

 where and occasionally take an important part in the constitution of 

 the mixed plankton. Among the Salpcc there are for example the 

 smaller species which form extensive swimming shoals. From the 

 three-year observations of Schmidtlein it is learned tliat the »aipas 

 belong to the perennial plankton and are numerous throughout the 

 wholeyear (19, p. 123). 



K.— Vertehhates 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 easi-ly 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 i)/fmA-fo/i against the neJdon is very 

 arbitrary and at any moment may be changed in fiivor 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 (pnintity may be increased ten or a hundred fold. In a compre- 

 hensive consideration of the plankton conditions, and particularly in 

 its physiological, ecological, and chorologieal 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 relation to the pur- 

 pose of this plankton study. We can here in Hensen's sense (9, p. 1) 



