34 PKOF. W. A. HERDMAN: COMPARISON OF SUMMER PLANKTON 



certain to have been taken at the former period, say, between the middle of 

 March and the middle of May. 



But in the Hebrides, as we have seen, very large phyto-plankton hauls may 

 be taken year after year in July — when in the Irish Sea the hauls are, for 

 the most part, comparatively small, and are all composed of zoo-plankton. 

 Mr. Andrew Scott, A.L.S.^ who has been associated with me for some years 

 in studying the plankton of the Irish Sea, remarked, when I showed him 

 some of the phyto-plankton samples from Cauna, Rum, and Ardmore : " If I 

 had not seen the locality and date on the bottles, I should have placed them, 

 without doubt, as Irish Sea gatherings taken in April.^^ And the resem- 

 blance, I may add, is not merely in general appearance, but extends to the 

 microscopic composition. The gatherings from Ardmore, for example, 

 contain abundance of Chcetoceros contortum and C. decipiens, Rliizosolenia 

 semispina, Lauderia borealis, Thalassiosira gravida, and T. Nordenshioldii — all 

 of them Diatoms that are characteristic of an April gathering in the Irish 

 Sea, off Port Erin. The abundance of the two species of Thalassiosira 

 makes this and other July gatherings from round Canna and Mull quite 

 unlike a September Diatom haul in the Irish Sen, as the genus Thalassiosira, 

 abundant in the North, is practically absent at the time of the autumnal 

 maximum in the South. 



In order to demonstrate still further the characters of these diverse 

 planktons, and illustrate the comparison between the Scottish summer 

 series and Irish Sea gatherings typical of different seasons, I give on p. 35 

 in tabular form the quantitative details * of : — 



(A) A typical coarse zoo-plankton from off Skate Island at the 



entrance to Loch Fyne, July 18th. 



(B) A zoo-plankton of somewhat different type (with fewer Calanus, 



but many more of the smaller Copepoda), from Kilbrennan 

 Sound, July 26th. 



(C) A mixed gathering, mainly phyto-plankton, from oft' Loch Ranza, 



Arran, July 27th. 



These three somewhat diverse samples were obtained, it will be noticed, 

 within a period of ten days, in July, from localities not ten miles apart. 



I add also, for comparison, the similar quantitative record of three charac- 

 teristic gatherings made in the centre of the Irish Sea, off the west coast of 

 the Isle of Man, near Port Erin, as follows : — 



(D) A typical phyto-plankton, taken in April — a large haul (100 c.c). 



(E) A typical scanty zoo-plankton, from August (2*5 c.c). 



(F) A mixed gathering, mainly phyto-plankton, in September, when the 



autumnal Diatoms are present in quantities (11 c.c). 



* The enumeration of the species in these quantitative lists was made by Mr. Andrew 

 Scott, A.L.S., for our joint report on the Irish Sea plankton. I am indebted to my friend 

 Mr, Edwin Thompson for the photo-micrographs from which figs. 5 and G were prepared. 



