34 PROCEEDIJfGS OF THE 



our results shows in some cases close resemblances between 

 adjacent stations (such as I and II) on the same day, or between 

 adjacent days at the same station, and in other cases just as 

 striking differences. How far these points of similarity and of 

 divergence are normal and are fundamental, or how far they are 

 due to wind, sun, and other weather conditions, or to tidal or other 

 currents, will require detailed consideration. 



A further point that has been brought out in the progress of 

 this investigation is the obvious distribution of at least some 

 organisms in swarms. This can occasionally be seen b}^ the eye, 

 when, for example, shoals of large Medusae are encountered which 

 are so abundant for a limited area that on a calm day they may 

 cover the surface Hke a tessellated pavement, and assume polygonal 

 forms from mutual pressure. On other occasions the nets have 

 evidently encountered swarms of Copepoda, of Cirripede Nauplii, 

 of Crab Zoeas, of worm larvae or of other organisms. One might 

 expect such results in the case of neritic forms, w"hich are merely 

 stages in the life-history of some gregarious organism ; but the 

 occurrence is by no means confined to such, it extends to oceanic 

 organisms on the high seas, and this sporadic distribution in 

 swarms has not been sufficiently taken into account by some 

 writers who have treated of the distribution of the plankton in 

 recent years. 



The Irish Sea contains a surprising number of w^hat are usually 

 regarded as " oceanic " species — not merely as occasional visitants, 

 but as normal and continuous constituents of the plankton during 

 a great part of the year. Amongst these may be mentioned 

 Chcetoceros densum, Coscinodiscus radiatus, Itliizosolenia semispina, 

 Ceratiuni tripos, Peridinium sp., Tomopteris onisciformis, Sagitta 

 hipunctata, Pleurobrachia pileus, Calanus Jielgolandicus, Anomalocera 

 pattersoni, Acartia clausi, Oithona similis, and OikojjJeura dioica. 

 Some of these oceanic species seem, so far as we can judge from 

 the published records, to be more abundant and more continuously 

 present round the Isle of Man than they are even in the \A'estern 

 part of the Enghsh Channel. 



We have evidence from our closing vertical nets that the zone 

 of most abundant life is not on the surface but is generally a few 

 fathoms below — sa)^ usually, between 5 aud 10 fatlioms. Samples 

 of water from 5, 10 and 20 fathoms obtained with the " Mill " 

 water-bottle support the above statement. But this conclusion was 

 arrived at and could be established, quite apart from the evidence 

 of the vertical nets, from a comparison of the results obtained by 

 the weighted and surface open horizontal tow-nets. At the time of 

 the Diatom maximum in spring, however, our closing vertical nets 

 showed that these Protophyta are more abundant in the deeper 

 zones than at the surface, and increase in density downwards to at 

 least 20 fathoms. 



In the cases of some groups, e. g. Cladocera and Oikopleura, 

 the distribution is sometimes remarkably regular, the same 

 numbers being taken simultaneously by comparable nets at 



