PLANKTONIC STUDIES. b97 



the new elosible net iji vented by Palumbo, be was enabled to bring up the 

 entire animals from deliiiite deptlis. From these experiments Cbiercliia 

 concluded ''that certain characteristic si)ecies of siphonophores live in 

 great numbers at certain depths, from 1,000 meters above the bottom 

 n]>wards, the strongest and most resistant in the depths, the vreaker 

 higlier up" (8, p, 80), Other siphonophores, wliich belong to the forms 

 most numerous at the surface, extend down to considerable depths, as 

 Diplujes sieholdii (15, p. 12). The larvai of Hippopodius luteus, which 

 are very numerous in winter and spring, have quite disap])eared iii 

 summer, and, according to Chun, live in greater deptlis, 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 

 Physonectw. From these and other grounds the participation of the 

 sijDhonophores in the plankton, like tiiat of their ancestors, the Hydro- 

 vieduscc, is extremely irregular, and their appearance at the surface of 

 the sea is subject to the most remarkable changes. 



Ctenopliores. — This Cnidarian class also, like the preceding, is purely 

 oceanic, not neritic. They also show the same phenomena of jielagic 

 distribution as the i<iplionopliores and 3h'dvsa\ frequent a])pearance 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 plankton ic 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 ctenoi}hores, which 

 in the spring are found as larvse at the surface, later sink, pass the 

 summer in the cjooler depths, and rise to the surface in the autumn in 

 crowds, as mature animals. The irregularity of their api)earauce is also 

 mentioned by Graeffe (20, j). 361). 



E. — Helminths of the Plankton. 



The race of the helminths or "worms" (the cross of suffering for sys- 

 tematic zoology) obtains a more natural unity and more logical defini- 

 tion, if one removes therefrom the platodes and annelids, placing the 

 former with the coeleuterates, 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 higlier 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, ]). 510). Tliere remain then as helminths, in the narrower 

 sense, four divisions with about 12 classes, namely, (1) the Rotatoriw 



