fiOO REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



fauna and do not occur in the ocean. They also are found in such 

 limited numbers tliat they are without importance for the i)lankton. 



Much more important for us are the free-swimming echinoderm larvae, 

 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 Midler, forty-five years ago first ap- 

 plied the method of "pelagic tishery 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 ui)on that of their 

 benthonic parents; but in addition also partly upon chorological, partly 

 oecological causes. According to Sir Wyville Thompson (14, ii, pp. 

 217-245; C, 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 tlie 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 larvae of these 

 animals occur commonly only in the neritic plankton of the temperate 

 and warm zones, not in tlie open ocean. Tliey 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 or 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 liave 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 ex(;eptionally), 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 acrpiired the ])e]agic mode of life and have assumed the 

 characteristic hyaline condition of the oceanic glasslike animals, the 

 swimming Tomopteridte and Alciopidm. Both families are represented 

 in the i)lankton only by a few genera and si)ecies, and as a rule their 

 THimber 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 rojtresented in the different depths, from 500 to 1,300 meters, 

 by peculiar zonary and bathybic species, whicli are distinguishable 



