Throughout a complete year from September, 1915, to September, 1916, sea-water samples were taken regularly two or three times a week from beyond the Breakwater in the region of the Knap buoy, 2½ miles from Plymouth shore, from the surface and at 5 and 7 fathoms, The object was to supplement the existing records from the tow nets as it is well known that a very large amount of material is lost even from the finest nets, as Lohmann has shown exhaustively (1908). So far the only plankton records from this region have been from the tow nets, and a glance at the tables given at the end of this paper will show directly, if compared with those by Gough (1903–7) and Bygrave (1911), also Cleve (1899 and 1900), the great difference in numbers of the smaller forms, or their entire absence from the tow nettings. Again, no actual numerical records have been given from this region. At the same time as the water samples were taken, tow nettings, coarse, medium and very fine, were also secured, and these were regularly examined for comparison.

The following list includes all the Peridiniales identified in the plankton throughout the year from September, 1915, to September, 1916, from the water samples, details of which will be found in another paper in the same journal (p. 133). Also from the plankton in the tow nets in the same year and a portion of the summer 1915. As is shown in the abovementioned paper, the summer is the time for nearly all the Peridiniales, June being the maximum month. After October very few are seen in the water samples, although in the tow nettings the larger and stronger forms, such as Ceratium and Peridinium, are still present.

Sagitta has been several times noticed as a host for various parasitic worms, notably a larval nematode and several trematodes. Larval cestodes have also been seen in it. It is exceeding voracious and apparently eats almost any animal food, especially its own species and small Crustacea, so that it is not to be wondered at if it forms a convenient intermediate host for many worms, and as Sagitta itself is an important fish food it naturally follows that the adults of these parasitic worms are usually found in fish as their final hosts. So far, however, the life histories of the larval forms hitherto found have not been determined, so we are pleased to be able to identify two trematodes belonging to well-known species which inhabit Sagitta as intermediate host and fish as the final host.

Table XLV. is perhaps of interest, as showing the composition of the catch obtained with the young-fish trawl at different times of the year. It has been obtained by combining certain groups of hauls made in 1914 in the offshore waters outside Plymouth, all of them being beyond the 20-fathom line. As far as the conditions are concerned therefore the different groups are fairly comparable. The figure given for each species is the average number of specimens per haul for the group. It will be seen that after June the number of species present as well as the average number per haul are both very much reduced.

The amount of phosphoric acid in sea-water off Plymouth was at a maximum of 0.06 mg. per litre of P2O5 at the end of December, 1915, after which it fell irregularly to a minimum of less than 0.01 mg., which extended from the last week of April to the latter part of May; it then increased again and in January, 1917, reached the same value as the average for the first part of the month in the previous year.

This seasonal variation is probably to be attributed to the removal of the phosphates from solution, at first by the fixed algæ, and later in the spring by the diatoms and for a short time by Phcœcyslis. There is also present in sea-water taken near Plymouth another soluble compound of phosphorus which can be converted into phosphoric acid by oxidising agents.