PLANKTONIC STUDIES. ; 597 
the new closible net invented by Palumbo, he was enabled to bring up the 
entire animals from definite depths. From these experiments Chierchia 
concluded “that certain characteristic species of siphonophores live in 
great numbers at certain depths, from 1,000 meters above the bottom 
upwards, the strongest and most resistant in the depths, the weaker 
higher up” (8, p. 86). Other siphonophores, which belong to the forms 
most numerous at the surface, extend down to considerable depths, as 
Diphyes sieboldii (15, p. 12). The larve of Hippopodius luteus, which 
are very numerous in winter and spring, have quite disappeared in 
summer, and, according to Chun, live in greater depths, even to 1,200 
meters (15, p. 14). Other forms are spanipelagic and come to the sur- 
face only for a short time, only a few weeks in the year, like so many 
-Physonecte. From these and other grounds the participation of the 
siphonophores in the plankton, like that of their ancestors, the Hydro- 
meduse, is extremely irregular, and their appearance at the surface of 
the sea is subject to the most remarkable changes. 
Ctenophores.—This Cnidarian class also, like the preceding, is purely 
oceanic, not neritic. They also show the same phenomena of pelagic 
distribution as the Siphonophores and Medusa, frequent appearance in 
great swarms, sudden disappearance for long periods, unaccountable 
irregularity in their participation in plankton formation. The tables 
which Schmidtlein has given on the basis of three years’ observa- 
tions, on their periodical appearance in the Gulf of Naples, are very 
instructive for all three classes of the planktonic Cnidaria (19, p. 120). 
The ctenophores also, up to a short time ago, were regarded as auto- 
pelagic animals; but of them also it has been discovered that they 
extend in abundance to various, somewhat definite depths. Chun, in 
his monograph of the ctenophores of Naples (1880, p. 236-238) has 
pointed out that these most tender of all pelagic animals have just as 
definite vertical as horizontal migrations. Many ctenophores, which 
in the spring are found as larve at the surface, later sink, pass the 
summer in the cooler depths, and rise to the surface in the autumn in 
crowds, as mature animals. The irregularity of their appearance is also 
mentioned by Graeffe (20, p. 361). 
E.—HELMINTHS OF THE PLANKTON. 
The race of the helminths or “‘ worms” (the cross of suffering for sys- 
tematic zodlogy) obtains a more natural unity and more logical detini- 
tion, if one removes therefrom the platodes and annelids, placing the 
former with the ccelenterates, the latter with the articulates. The jus- 
tice of this limitation and also the grounds for regarding the worms as 
the common ancestral group of the higher animals, I have set forth 
already in the “Gastrea Theory” (1873), and many times at later op- 
portunities, last in the eighth edition of my “Natural History of Crea- 
tion” (1889, p. 540). There remain then as helminths, in the narrower 
sense, four divisions with about 12 classes, namely, (1) the Rotatorie 
