PLANKTONIC STUDIES. 697 



the new closible net invented by Pahinibo,]ie was enabled to bring up tlie 

 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" (S, p. 80), Other siphonophores, which belong to the forms 

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

 Dijyhyes sieholdU (15, p. 12). The larvae of Hijjjyopodius luteus, which 

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

 Physonecta'. From these and other grounds the participation of the 

 siphonophores in the jDlankton, like that of their ancestors, the Hydro- 

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

 the sea is subject to the most remarkable changes. 



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

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

 distribution as the Siplionophores and Medusa', frequent ajipearance in 

 great swarms, sudden disappearance for long periods, unaccountable 

 irregularity in their particifjation 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 plauktonic Gnidaria (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. 230-238) 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 larvae at the surface, later sink, pass the 

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

 crowds, as matnre animals. The irregularity of their appearance is also 

 mentioned by Graefte (20, p. 361). 



E. — Helminths of the Plankton, 



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

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

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

 former with the c<elenterates, 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, ]). 510). There remain then as helminths, in the narrower 

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



