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Pari III. — Fifteenth Annual Report 



vegetation of the ocean, it will be at once apparent that the small fringe 

 of visible vegetation on the shallow bottoms round coasts cannot in the 

 least degree suffice for the sustenance of the teeming animal life, which 

 not only extends over the surface, but ranges into the depths. This 

 role must be played almost entirely by the minute plant organisms, diatoms, 

 Protococcacete, Oscillatoriece, Peridiniece, Coccospheres, Rhabdospheres, 

 etc., which inhabit the surface layers down to 30 to 50 fathoms. Such 

 organisms have been recorded from polar, temperate, and equatorial seas, 

 sometimes in vast shoals, discolouring the water, but always found to be 

 present when suitable apparatus is used for their capture. Their economic 

 importance to the fisheries is therefore of a direct and vital character. 

 We know that the stomachs of Holothurise, Ascidiaris, Salpoe ; oysters, 

 scallops, whelks, and other molluscs ; crabs, lobsters, and other large 

 Crustacea, and even full grown fishes, have formed the happy hunting 

 grounds of diatomists in search of material ; but direct evidence has 

 been wanting of the use of diatoms as a matter of daily food by animals 

 in the sea. Observing that the excreta of copepoda and other small 

 Crustacea, largely fed upon by fishes, were in many cases tinged 

 with a faint colour like diatomine, I subjected them in many 

 cases to a minute microscopic analysis, with the result that nearly the 

 whole could be resolved into minute fragments of diatom frustules and 

 their chromatophores. In many cases, it was surprising to note that 

 the chromatophores had passed through, almost unchanged in shape, 

 and even retaining faintly their colour. It was, in fact, often possible, 

 from the characteristic shapes of the chromatophores, and the minute 

 finely sculptured fragments of frustules, to recognise them as belonging to 

 diatoms found in the same capture. In a few cases it was possible to 

 detect a whole unbroken diatom cell within the crustacean, but almost 

 invariably the diatoms were reduced to fine fragments. Coscinodiscus 

 and Skeletonema were the usual forms found, and these were the preva- 

 lent forms in the sea when my observations were being made. 



It appears, then, to be clear that the animals which are themselves an 

 important constituent of the food of fishes live in turn largely on diatoms. 

 It has always seemed to me very highly probable that young fishes eat 

 diatoms directly ; and, in order to put this to the test, Dr Weniyss Fulton 

 sent me some young sand-eels, taken in tow-net, 15 miles off Aberdeen, 

 on 16th May 1894; some young flat-fish (? plaice), taken off Montrose, 

 21st May 1894; and some very small clupeoid fishes, taken 30th March 

 1889. They were preserved in spirit. After burning a few of each in 

 platinum crucibles, the ash was examined, and diatoms discovered in 

 every case. Four different genera, viz., Skeletonema, Eucampia, Melosira, 

 and Chmtoceros, were found in the sand-eels ; Skeletonema and Nitzsclda 

 in the flat-fish (in both cases Skeletonema was predominant) ; and Coscino- 

 discus (abundant) and Melosira (very rare) in the young clupeoids. In 

 each case, complete valves (in Skeletonema, chains of several cells) were 

 found, which appears to indicate that they were eaten directly by the fish 

 themselves, and not within the bodies of small Crustacea ; in this latter 

 case, they would have been broken up into minute fragments.* 



* The late Mr R. Brown in a paper " On the Nature of the Discoloration of the 

 Arctic Seas" {Trans. Bot. Soc, Edin., vol. ix. p. 244, 1868), was the first, so far as 

 I know, to point out that this discoloration is due the presence of large masses of 

 diatoms. He found these diatoms within the Pteropoda, Medusae and Entomostraca 

 that compose the " whale's food." 



With reference to the association of diatoms with the food of fishes, Prof. 

 M'Intosh refers me to the Seventh Annual Report of the Board, in which he 

 anticipates my observations. At p. 272 (Part III.) he notes the occurrence of 

 Appendicularians with Rhizosolenia. He says, ' ' The stomachs of these were distended 



