The larvæ of the Decapod Macrura occur throughout the year in the tow-nettings taken in Plymouth Sound; while during the summer months they form an important and conspicuous feature of the coastal plankton.

In only a few cases have their life-histories been fully worked out, owing probably to the difficulty of rearing the larvæ under artificial conditions; and the literature on the subject is consequently both scanty and scattered. It has therefore been suggested that a brief review of the larvæ already described, together with a few additional ones which have been identified during two years' examination of the Plymouth plankton, from 1917–1919, might be of some use to future investigators.

The object of the present paper is to enable future workers to see at a glance how far these forms have been investigated, and especially to facilitate the identification of the living larvæ from the tow-nets.

No attempt has been made to describe the appendages in detail, as such particulars can generally be found in the literature cited. A general outline drawing is given of the more important forms in their first larval stage, and a figure of the telson and uropods in the fourth or last larval stage is also shown.

As the coloration of the larva is often of great assistance in identifying the living animal, it is here described rather fully, and, whenever possible, the drawings and the colour notes have been made from the living larva. In cases where no direct observations were available the colour descriptions are quoted from other authorities.

The young clupeoid material which had been collected from 1913 onwards and sorted out with a view to future biological study, especially of the Pilchard, was handed over to me by Mr. R. S. Clark with the main object of differentiating the three species Pilchard, Herring and Sprat. A research was therefore undertaken to find out the best way of distinguishing them through all their life history until metamorphosis. This was practically finished when the thesis by Fage (1920) on young clupeoids of the “Thor” expedition was published, rendering a large part of my work unnecessary, as, thanks to him, it is now almost impossible to mistake them at any period of their development, and they are clearly differentiated at any time in their life history.

Whilst investigating the food of young fishes in general (Lebour, 1918–1919–1920) some thousands of young Clupeoids were examined, with the result that by far the larger proportion of post-larvæ were found to be empty. Here and there, however, food was present, and by bringing these records of 1917 to 1919 together, with some from 1920 and 1921, we have a good idea of the food of our three common Clupeoids—Herring, Sprat and Pilchard—in their young stages.

A probable reason for this apparent absence of food in the transparent stages is that the food must be very quickly digested on account of its being so conspicuous; thus it is probably taken only at long intervals and rapidly got rid of. It would follow that under these circumstances we should only occasionally catch the fish when food was inside it.

The observations on the food of the young Herring are perhaps the most interesting of the three on account of the fact that it eats solid food some time before the yolk sac is absorbed, the mouth being open when the fish is hatched, whereas the Sprat and Pilchard, being much less advanced, are only able to feed on outside food much nearer the time when the yolk sac disappears. Unfortunately we have no records of Pilchard food in the very young stages (under 12 mm.), green food remains being found in those of the Sprat before the yolk sac has completely disappeared.

In view of the fact that dogfishes are now of undoubted importance as marketable food fishes, it is necessary that the life-history of each of the species concerned should receive closer attention than hitherto. The present publication is the result of an endeavour to gain a personal working knowledge of the general life-history of the species which are landed at Plymouth. In carrying out a scheme of work of this kind it is certain that many of the observations made will have already been carried out previously by other workers, but, nevertheless, the results obtained are of value, not only in regard to the degree of confirmation which they afford, but also in the building up of an account of the life-history under the local conditions. In some instances, too, it will be seen that the actual numeric results in themselves are of little statistical value, but they yield very interesting suggestions which may be confirmed or disproved by a repetition of the experiments on a larger scale. I express my thanks to the fish-buyers at Plymouth fish-market, particularly Mr. J. Vanstone, for their kindness in allowing me to handle their fish before dispatch, and for their assistance in obtaining a regular supply of material for many months.

Since 1912 extensive investigations have been carried on in the Laboratory of the Marine Biological Association at Plymouth on the genetics of Gammarus Chevreuxi Sexton, a brackish-water amphipod (Allen and Sexton '17 and '20, see also Sexton '13 and Sexton and Matthews '13).

During the course of the work, numerous individuals have appeared showing abnormalities in sexual behaviour or structure. In view of the interest attaching to such forms, especially after the work of Goldschmidt ('20), Banta ('16), Sturtevant ('20) and others, it was deemed advisable to collate and publish the available data in spite of their some what fragmentary character.

In normal circumstances, the males of Gammarus chevreuxi mate with the females some days (3 to 9 days) before fertilisation is due to take place. Unless the female is mated for the first time she will normally be carrying in her brood-pouch a brood of developing embryos. After hatching, the young are carried about 24 hours before extrusion, and during that time they come to the anterior end of the brood-pouch and feed on what the mother is eating. Finally they are extruded by a voluntary opening of the brood-pouch. Immediately afterwards the female moults, whereupon fertilisation occurs and a new batch of eggs is extruded into the pouch.

The pair have by now separated, and remain separated usually until the embryos have gone through about half their development in the pouch.

Cannibalism is of fairly frequent occurrence, the male eating the female immediately after she has moulted. It is extremely rare for the normal female to eat a male.

When shore-collecting for marine algæ at Plymouth in early April, an organism which has now been identified as a calcareous lichen was brought to my notice by Dr. Orton. It occurred in large patches on the rocks both above and below high-water mark in the neighbourhood of the Marine Biological Station, and was also very frequent on the limpets and barnacles in the vicinity. When examined with a lens, the thallus of the lichen was observed to be thickly dotted either with perithecia or with well-marked pits which had apparently been previously occupied with fruits. The lichen was found to be identical with the species collected in Robin Hood Bay, Yorkshire, by E. M. Holmes, in 1901, and described by A. Lorrain Smith as Arthopyrenia foveolata. Arthopyrenia differs from Verrucaria in the fact that the associated alga is Trentepohlia, and the above species is easily distinguished by the pits which occur on the thallus.

The same species has been recorded by M. C. Knowles from various places round the coast of Ireland, including the western shore of Blacksod Bay. It occurred on limestone rocks at about high neap-tide level, and also on live and dead barnacles where it was almost always associated with colonies of Rivularia atra. At Plymouth, the frequent occurrence of the lichen on limpet shells is a conspicuous feature in the distribution.

Four quarterly meetings of the Council and one special meeting were held during the year, at which the average attendance was fourteen. A committee of six members of the Council visited and inspected the Plymouth Laboratory. The thanks of the Association are due to the Council of the Royal Society, in whose rooms the meetings have been held.