600 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 
fauna and do not occur in the ocean. They also are found in such 
limited numbers that they are without importance for the plankton. 
Much more important for us are the free-swimming echinoderm larve, 
which often play a great part in the neritic plankton. Indeed they are 
classical objects in the history of plankton investigation; for to their 
study their discoverer, Johannes Miiller, forty-five years ago first ap- 
plied the method of “pelagic fishery with the fine net,” which soon led 
to such remarkable and brilliant results. The distribution and number 
of the larval rayed animals is naturally dependent upon that of their 
benthonie parents; but in addition also partly upon chorological, partly 
cecological causes. According to Sir Wyville Thompson (14, II, pp. 
217-245; 6, p. 379), the remarkable metamorphosis, discovered and de- 
scribed in a masterly way by Miiller, is the rule only among the littoral 
forms, chiefly in the temperate and warm zones; on the other hand, it 
is the exception in the case of the majority, for star animals of the 
deep sea and cold zones, in the Arctic as well_as in the Antarctic, 
develop directly. Therefore, great troops of pelagic larve of these 
animals occur commonly only in the neritic plankton of the temperate 
and warm zones, not in the open ocean. They seem to visit the depths 
(below 100 meters) very seldom (15, p. 17). Besides, their appearance 
is naturally connected with the time of year of this development; often 
only during a few months (9, p. 62). The variation in the constitution 
of the ‘periodic plankton” is here very remarkable. 
H.—ARTICULATES OF THE PLANKTON. 
Of the three chief divisions which we distinguish in the group of 
articulated animals (30, p. 570) two, the Annelids and Tracheates, take 
no part in the constitution of the plankton. Both are represented only 
by a few pelagic genera, and these have a limited distribution. Much 
greater in importance is the third chief division, the Crustacea. It is 
the only animal class which is. never lacking in the tow-net collections 
(or only very exceptionally), and which commonly appears in such 
numbers that their predominant position in the animal world of the 
sea is evident at the first glance. This applies as well to the oceanic 
as to the neritic fauna, to the littoral as to the abyssal benthos. 
Annelids.—The great mass of this group, so rich in forms, belongs to 
the benthos, and is represented in the abyssal as well as in the littoral 
fauna by numerous creeping and sessile forms. Only very few ringed 
animals have acquired the pelagic mode of life and have assumed the 
characteristic hyaline condition of the oceanic glasslike animals, the 
swimming Tomopteride and Alciopide. Both families are represented 
in the plankton only by a few genera and species, and asa rule their 
nuinber of individuals is not very considerable. Chun has lately 
shown by means of the closible net that both forms, Tomopteris as well as 
Alciope, are represented in the different depths, from 500 to 1,300 meters, 
by peculiar zonary and bathybic species, which are distinguishable 
