The family Raiidæ belongs to the division Batoidei of the sub-order Hypotremata, and is represented by the principal genera Raia, Psammobatis, and Sympterygia (Regan, 1906). The genus Raia (an altered spelling of Raja) was instituted by Linnæus, after Artedi, for the type Raja batis in the Systema Naturæ, Ed. X, Vol. I, 1758. The species are numerous and mostly of northern distribution.

Following the researches on the food of young fishes, it was thought advisable to investigate the food of the planktonic invertebrates. Whenever possible, therefore, the food of the larger animals brought in by the tow-nets was noted, and also that of many of the smaller creatures down to some of the unicellular organisms, such as the tintinnids and those members of the Peridiniales which are holozoic.

With the larger animals it was hoped to find which of them actually ate the young fishes, and by investigating their food in general ascertain how much they were actually competitors with the fishes.

The present work is offered as a preliminary, and it is hoped to continue it, following it up especially with more experimental work on the living animals.

The tow-nettings were examined fresh and the food noted. Sometimes a few hours elapsed between the taking of the sample and the examination, so that some of the catch was moribund in the jar. There is always the objection that the food might have been taken in the jar whilst the plankton was being brought in, and it is a matter of general observation that many medusæ and various pelagic animals will devour young fishes and almost anything living when crowded up with them in the hauls. In many of the organisms examined, however, the nature of the food was so consistent in various hauls and from various localities that it seems almost impossible to believe that it is merely accidental, and it is probable that what food one usually finds inside any planktonic animal is natural to it.

The investigation of the food contents in the stomachs of young fishes was included in the scheme of scientific investigations drawn up and initiated by Professor Herdman for the Lancashire Sea Fisheries Committee nearly thirty years ago. The lengthy series of Annual Reports contain here and there accounts of the observations made on the stomach contents of various Pleuronectidæ captured close inshore, and the pelagic stages of other fishes caught from time to time in the plankton tow-nets. No systematic attempt has, however, been made, in connection with the investigation of the Irish Sea, to determine the food of any particular species of fish during the early part of its life history.

Other observers working in other areas, notably Dr. Marie Lebour at Plymouth, have added very much to our knowledge of the early food of young fishes. Dr. Lebour's reports, published in the Journal of the Marine Biological Association, Vols. XI and XII, deal with a very large number of larval and post-larval stages of the more important food fishes caught in the tow-nets and young fish-trawl in Plymouth Sound and beyond.

The present report gives an account of the food contained in the stomachs of young plaice (Pleuronectes platessa, Linn.) from a few days after hatching to about five months old. The samples examined in April and May were taken from the spawning pond at Port Erin, Isle of Man, where they had hatched from the pelagic eggs spawned by the adult plaice early in 1921. The later stages examined during May to August represented young plaice hatched in the open sea about the same time as those in the pond, and which had made their way close inshore.

The post-larval blennies taken off Plymouth during recent years have been assigned to three species, namely, Blennius ocellaris L., Bl. pholis L., and Bl. gattorugine L. The process of identification, however, has emphasised the need for the revision of the available descriptions and illustrations of the various stages in development, and I have, therefore, thought it desirable to report on the Plymouth forms and give drawings of them.

Blennius Ocellaris (12 rays to the pectorals).

Ehrenbaum (2, p. 83) has copied two figures of the young of this species which were originally produced by Holt (5, p. 45), and it is somewhat unfortunate that both need to be commented upon. The first is satisfactory as a representation of the appearance of a larva some time after hatching, but the length is wrongly given as 6.3 mm. Garstang (4, p. 74) explained that this was an error in transcription, and, after a re-examination of Holt's specimens, he found that the total length was 4.6 mm. (varying between 4.55 and 4.65). The second figure, depicting a young fish of 18 mm., which Holt tentatively assigned to Bl. ocellaris, was taken exception to by Garstang and the identification questioned, and it must be admitted that it does not portray satisfactorily the characters of the species.

The full account of the rearing of the young from the egg by Garstang (4) was unfortunately not illustrated; but as several of his original specimens are in the Plymouth collection, I have been able to identify quite readily the more recently captured examples and have made a series of drawings which are given below (Figs. 1 to 4).

Six species of Wrasses occur in the adult stage in the neighbourhood of Plymouth (Clark 3, p. 217), namely, Labrus bergylta Asc., Labrus mixtus (L.), Ctenolabrus rupestris (L.), Crenilabrus melops (L.), Centrolabrus exoletus L., and Julis julis (L.).

In the spring of 1920 the Ministry of Agriculture and Fisheries approached the Marine Biological Association of the United Kingdom with a view to the Association undertaking the manufacture of a large number of “Drift Bottles,” to be used in an extensive research into the resultant movements of the waters of the North Sea.

It has long been known that sea water is alkaline and numerous determinations of its alkalinity have been made. The method adopted was the usual one for mixtures of carbonates and bicarbonates, or some modification of it. Those waters which give no colour with phenolphthalein contain bicarbonate only, but for the most part ocean waters have a small amount of carbonate also. Owing to the presence of larger amounts of carbonates and bicarbonates the reaction of sea water is more stable than that of rain or river water, inasmuch as it has a greater alkaline reserve which acts as a “buffer.” The significance of this has been pointed out by Moore, Prideaux, and Herdman (1915) and by other workers. The measurement of alkalinity was carried out by the above named using N/100 hydrochloric acid and titrating to the end points with phenol phthalein and methyl orange. The results are recorded in cubic centimetres of centinormal acid per 100 c.c. of sea water; this is convenient as it is what is measured directly, but others adopt the perhaps more rational notation of milligram equivalents of hydroxyl per litre (Buch, 1914). One cubic centimetre of N/100 acid per 100 c.c. corresponds to 0.1 milligram equivalent per litre. Some workers on fresh waters, Birge and Juday (1911) for example, consider water as acid if it contains more carbon dioxide than that sufficient to convert the carbonate into bicarbonate, and titrate back to a pink with phenolphthalein. Their acid water is, however, still alkaline to methyl orange.

1. On storing, sea water suffers a decrease in pH value. The amount of change varies from that produced by adding 1·0 c.c. of N/100 acid to 100 c.c. of sea water, up to that due to adding about 2·5–3·0 c.c. The decrease is due to the production of carbonic acid by organisms.

2. The change corresponding to 1·0 c.c. of acid, as above, is equivalent to that produced by the complete oxidation of 3 milligrams per litre of a hexose sugar, which requires 3·2 mgrms. per litre of oxygen. This is the minimum value, from 8·0–9·6 mgrms. corresponds to the higher values of acid. Figures for oxygen consumption by estuarine waters, which are available for an approximate comparison, show that 1·5 mgrms. of oxygen is not often exceeded in estimations by means of alkaline permanganate. The highest of this series is 5·6 mgrms. It is suggested that the different results given by the two methods are due to the fact that respiratory changes taking place in the water during storage set free much of the organically combined carbon before the oxidation by permanganate has been started. Determinations on freshly drawn filtered sea water give, according to Raben, 7·5 mgrms. of hexose or 8·0 mgrms. of oxygen consumed.

3. It is probable that the change in pH value on storing indicates the amount of plankton present, at any rate when sewerage contamination is negligible. It appears that water near the surface, at 20–25 metres and sometimes at the bottom, 70 metres, is particularly subject to change during storage. Four cases out of seven showed marked decreases in pH value at 20–25 metres, and two others exhibited the change to a less marked degree.

4. The total amount of carbon, reckoned as hexose, which is set free during storage by respiration in sea water at E1 is about twice that photosynthesised between July and December, taking for this the minimum value 3 mgrms. per litre, namely a total of 6 mgrms. per litre. Considering the column of water from bottom to surface, this is equivalent to about 500,000 kilograms per square kilometre in the English Channel off Plymouth.

1. Brom thymol blue may be used in dilute solution for ascertaining the hydrogen ion concentration of certain marine organisms. It penetrates slowly, but the stained portions remain actively motile, so its toxic action does not appear to be great at the dilutions found serviceable.

2. The animals studied gave values from pH6·2 to about pH7·5, though possibly the more alkaline end of the range may be pathological. About pH0·2 should be subtracted from these figures for neutral salt error. The sea water used was initially at pH8·2, corrected.

The measurements recorded for marine algæ of various groups show that the reaction of the sap is in most cases almost neutral, and in no case is the sap of the pronounced acid character met with in many land plants. This being so it follows that the enzymes concerned in the metabolism of these algæ must be quite different from those which effect corresponding changes in land plants, as may be seen on referring to the optimum pH values for various enzymes quoted in the writer's previous paper on the reaction of plant cells (1922).

Sea water originally at pH8·2 became as alkaline as pH9·4 by the photosynthetic action of Ulva in removing carbonic acid. This degree of alkalinity did not prove injurious to Ulva, but exposure to it for two and a half hours at 27° C. sufficed to increase the permeability of the superficial cells of Ceramium rubrum irreversibly and fatally. Similar though less rapid changes brought about the death of numerous Vorticellæ epiphytic on Ceramium. The criteria for living cells were permeability to neutral red and impermeability to brom cresol purple. Dead cells do not retain neutral red specifically and are permeable to brom cresol purple.

It is suggested that the above facts have a bearing upon the distribution of these and similar algæ upon the shore.

Formalin, which is permanently non-acid and only slightly alkaline, close to pH9, may be prepared by the addition of borax till a good red colour is shown with phenolphthalein, or a slaty blue with thymol blue, when added to the diluted formalin.

Distillation of formalin from solid magnesium carbonate gives an acid product, which is at pH4·4.

Formalin neutralised with sodium hydroxide becomes acid on standing, the change being more rapid in sunlight than in the dark. A reaction of pH5·2 was reached in fifty-six days in light, the initial value being pH8·0. Commercial formalin, “ 40 per cent,” may be as acid as pH2·8.

Much discussion has recently taken place over the peridinian described by Bergh (1882) as Diplopsalis lenticula, and certainly three distinct species have severally been designated as the original. As it is a name frequently to be found in the International Plankton lists, it is important that we should be clear as to which species we are noting, and as recent research at Plymouth has shown that all the three are common in the district it seems worth while to investigate them once more. The question is interesting also from the point of view of phylogeny, as in these species we have close relatives of Peridinium proper which seem to indicate the method of evolution of that genus from simpler forms with fewer plates, and from this point of view alone they are well worth study.

In centrifuging samples of sea water from Plymouth Sound and some of the stations outside, some interesting forms of Exuviella were found. These are all minute, the largest measuring 22 µ in length, and consequently are easily overlooked, although one at least is abundant during the summer months. This species, which is the commonest Exuviella in the Plymouth district, has been identified as Exuviella perforata Gran, described by him from the North Sea (1915), and is a new record for the English Channel. As it appears to be very little known and its structure is unusual in several particulars, the following notes are not without interest.

In 1915 Gran (page 99, Fig. 7) describes and figures Exuviella perforata, a new species from the North Sea at a depth of 0–20 metres, occurring in the eastern part with a density of 100–760 specimens per litre. It is roundish oval or nearly circular in shape, measuring 22.5–25 µ long and 21–22.5 µ wide, with a broad girdle and thick shell (thickness of cell 14–17 µ). The character, however, that gives it peculiar distinction is the depression in the centre of each valve, as Gran puts it, “with a sharply confined point-shaped perforated deepening in the centre.” This separates it from any previously known species of Exuviella. The cell contents owing to contraction could not be described in detail, but in a footnote it is stated that living specimens from Arendal in March, 1914, had brown chromatophores. Cleve-Euler (1917) records an Exuviella, which he says may be E. perforata Gran, as occurring frequently in the Skaggerak, from 0 to 100 metres.

In centrifuged water and in very fine tow-nets there sometimes occurred an extremely small species of Phalacroma, which appears to be new.

The genus Ilyanthus was erected by Forbes (1840) for I. scoticus; little is known about that species, and its anatomy is undescribed. There is no evidence that any subsequently described species is really a genuine Ilyanthus, and the genus-name can only be used provisionally for others until I. scoticus can be dissected. Meanwhile the genus-name must be reserved for I. mitchellii, the main subject of this paper. Gosse described this species from a specimen which he saw in 1853 (see Actinologia, p. 232); it must have been in poor condition or semi-contracted, and his figure (PL. 8, Fig. 6) is quite misleading. Andres erected (1883, p. 462) a genus Mesacmœa for his Ilyanthus stellatus of 1880, and, as I hope to show below, M. stellata is either specifically or at any rate generically identical with I. mitchellii; so that Mesacmœa now becomes a synonym; if it should prove, later on, when I. scoticus can be examined, that I. mitchellii does not really agree with it, i.e. is no Ilyanthus, then the name Mesacmœa can be revived for mitchellii and stellatus. Carlgren (Actiniaria of the Danish Ingolf—Expedition, 1921) has given a few anatomical details of Me acmœa, gained from notes by Andres in his possession; they agree with my description of I. mitchellii as far as they go, but there are not enough of them to make a full understanding of the genus possible. In Andres' 1883 monograph will be found a description of another species, I. partenopeus (p. 459).

Parorchis acanthus is a Trematode described by Nicoll (1907) from the bursa Fabricii and rectum of the Herring Gull, Larus argentatus; in 1906 he first described it as Zeugorchis acanthus. Originally found at St. Andrews, it is now known to be common at Millport and also occurs in gulls from the Northumberland coast, and probably is abundant elsewhere. Nicoll found it once in the Common Gull, Larus canus. In 1907 (Lebour, 1907) one of the present writers described a cercaria occurring in rediæ from Purpura lapillus. This was named Cercaria purpurœ sp. inq., and afterwards identified with young stages of Parorchis acanthus in the Herring Gull (Lebour, 1914). It was thus shown that the first host of this Trematode is Purpura lapillus and the final host the Herring Gull, but the intermediate host was unknown. Localities for the cercaria were Loch Ryan, Wigtownshire; Budle Bay, Fenham Flats and Cullercoats in Northumberland; Robin Hood's Bay in Yorkshire; and Millport. Adults occur at St. Andrews, Northumberland coast and Millport, as already stated.

The Cercaria appeared to be closely related to Echinostomum cercariæ, and it was suggested (Lebour, 1914) that the intermediate host would probably be some marine bivalve, as it is often so with Echinostomum (e.g. E. secundum in Mytilus edulis and Cardium edule, E. leptosomum in Scrobicularia tenuis). This suggestion now proves to be correct, and we are able to show that the intermediate host of Parorchis acanthus is Mytilus edulis or Cardium edule.

On August 13th, 1921, at the Millport Station some larval cercariæ were noticed by Elmhirst swimming in a small glass aquarium, which contained a collection of Purpura lapillus, Cardium edule and Mytilus edulis.

When collecting on the shores in the Plymouth area, while a member of the Easter Class at the Marine Biological Laboratory, Plymouth, in 1921, I was struck by the absence above low-water mark of Echinus esculentus, as contrasted with its abundance in this zone at Port Erin in the Isle of Man; for there, as Chadwick states, “it may be collected by hand on the beach, and on the ruined breakwater at low-water of spring tides.” There is no record of this species except below tide-marks in the “Plymouth Marine Invertebrate Fauna” list (Journ. Marine Biological Association, Vol. VII, 1904).

In 1849 W. P. Cocks, in the Trans. Penzance Natural History and Antiquarian Society (“List of Echinodermata procured in Falmouth and Neighbourhood from 1843 to 1849”), recorded the occurrence of this species thus:—

“Echinus sphœra.—Trawl refuse; common: young specimen found attached to stones, low-water mark.”

Later (1887–8), in the same journal, G. F. Tregelles, in a paper on “Echinodermata of Mount's Bay,” makes the following statement of the species of the genus Echinus: “The commonest and largest is E. esculentus (Pennant) [E. sphœra Forbes], which literally swarms off this coast at all depths. It is brought in by trawlers; it is found in crab-pots, into which it climbs laboriously after bait; the seaweed gatherers obtain them in from one to two fathoms of water. The Mouse-hole fishermen call them ‘zarts,’ doubtless an old Cornish word.”

Four ordinary meetings of the Council were held during the year, at which the average attendance was fourteen. A formal inspection of the Laboratory was carried out on behalf of the Council by Dr. G. P. Bidder, the other members of the Committee appointed for the purpose being prevented from travelling to Plymouth by a threatened railway strike. The thanks of the Association are due to the Council of the Royal Society, in whose rooms the meetings have been held.

Allen, E. J. Regeneration and Reproduction of the Syllid Procerastea. Phil. Trans. Roy. Soc., B. Vol. CCXI, pp. 131–177.