The present report deals with the young fish which were collected in the neighbourhood of Plymouth during the summer and autumn of 1919, from the beginning of July to the end of September, by small meshed nets operated from the Association's steamer Oithona. It is a supplement to Dr. Allen's report on the “Post-larval Teleosteans collected near Plymouth during the Summer of 1914,” published in Vol. XI, No. 2 (May, 1917), of this Journal, and to the writer's “General Report on the Larval and Post-larval Teleosteans in Plymouth Waters” in Vol. X, No. 2 (June, 1914).

The total number of fish examined amounts to 16,400, which are referable to 24 families, 39 genera and 71 species. All the forms are stages in the life histories of fishes, which are well known as adults.

The pelagic young of the species of Gadus and Pleuronectes are poorly represented in the catches, but this is not surprising when one considers the lateness in the year when the hauls were made. Similarly the young of early spawners like the Herring are absent.

The pelagic young of the Hake (Merluccius merluccius) appear for the first time since investigations with small meshed nets have been undertaken in the Plymouth area, the only other records of their occurrence in the western area of the English Channel being those which were taken by the Danish research steamer Thor, vide Schmidt (1909).

The post-larval stages of Ammodytes species captured in the young fish trawl and small-meshed nets during the cruises of s.s. Oithona in the year 1919 were separable into two well-defined series, A and B respectively. Details of the hauls of the trawl and nets with dates and localities are to be found on pages 210–212 of Mr. R. S. Clark's account of the cruises in the present volume.

Considering the adult forms of Ammodytes known to occur in Plymouth waters, it is observed that Ammodytes lanceolatus, Lesauvage, and Ammodytes tobianus, L. are frequently met with, but no record is to hand at present of the occurrence of Ammodytes cicerellus, Rafin., although the latter has been taken frequently in the Atlantic. In regard to the larvæ and post-larvæ, none comparable to those assigned to A. cicerellus by Louis Fage (1, p. 17) have been described previously as occurring in this district, nor have such forms revealed themselves in the material under present consideration.

In endeavouring to identify the post-larvæ of any species, a study of the characters of the adults is often of great help, but in the case of Ammodytes species the only features of the adult which seem to be of value in this respect are the number of vertebræ and the relative length of the head to the total length of the body, and even so these characters, on account of the difficulty of determining them, are not so helpful as they would appear to be. Other characters have therefore to be sought for, and the following are those by which Series A and B have been distinguished:—

On September 17th, 1919, a specimen of Leptocephalus Congri vulgaris (L. Morrisii) was captured in “bottom” haul No. 69 of the young fish trawl in locality Eddystone bearing S. by W. (magnetic) 1½ miles, over a depth of 31 fathoms. It was alive and active when taken, measuring between 115 and 120 mm. in length, and has been kept alive up to the present date in a glass aquarium under circulation. It was at a fairly advanced stage of development, intermediate between those exhibited in Figs. 2 and 3, Pl. I of Schmidt (1), the dorsal fin commencing slightly farther back than in Fig. 3, and the median row of pigment spots along either side of the body not extending forward beyond about 1 cm. in front of the commencement of the dorsal fin. There is a close resemblance also to the specimen figured by Fulton (2), with the exception that, as just pointed out, the median row of pigment along the body on either side does not reach forward quite to the head, and in addition the pigment spots at the base of the dorsal fin rays are only two or three in number, situated at the extreme posterior end of the fin.

Since the date of capture, observations 'have been made periodically and certain measurements taken as accurately as possible, but on account of the movements of the fish the latter could only be obtained approximately.

The eggs of Gobius minutus and Gobius microps are already known, Petersen (1892, 1917) having distinguished between them and figured them. Apparently the only mention of those of Gobius pictus is by Holt and Byrne (1901) who give the size of the eggs as about 0.80 mm. high by 0.65 mm. broad. Gobius minutus and pictus often occur together in the Sound and at the mouth of the Cattewater, but pictus is chiefly to be found near coarse sandy bottoms such as one gets near Duke Rock and New Grounds. Gobius microps occurs usually higher up the rivers, but the eggs and young are to be found occasionally in the Sound in the same localities as minutus and pictus.

Shells with eggs on them are commonly dredged in the Sound, those between Plymouth and the Breakwater are usually minutus, but pictus eggs are dredged from New Grounds and microps sometimes in the Catte-water.

In order to distinguish the eggs of these three species, and especially to obtain the eggs of G. pictus, adults of all three species were isolated in separate tanks with shells on the bottom on which the fish laid their eggs. The adults of minutus were from the Cattewater, those of pictus from New Grounds, and microps from Chelson Meadow, which opens into the Laira and where the water varies much in salinity, from almost fresh to ordinary sea-water strength. G. minutus and pictus were put into the tanks into ordinary running sea-water, straight from the vessel in which they were brought in from the dredge.

The investigation of the food of larval and post-larval fish from Plymouth Sound and beyond for three years has brought out some interesting facts. For the first two years fresh material from the tow-nets was examined together with old material from the Young Fish Trawl (Lebour, 1918, 1919). During the present year, in addition to the tow-nettings, fresh material from the Young Fish Trawl was also available from July to the end of September, and several fish from outside the Sound examined which had not been obtainable for the last two years. We thus have a fair number of young Mackerel, Ling, Hake and Gurnard and several others.

Inw “An Account of the Researches on Races of Herrings carried out by the Marine Biological Association at Plymouth, 1914–15,” Dr. J. H. Orton stated that the alimentary canal of each fish investigated was preserved for the future examination of the pyloric cæca. The present note gives the results of my counting of the latter, and the accompanying table on page 328 shows the number of pyloric cæca for each individual fish enumerated in Tables III and IV of the above publication.

Together with the specimens of C. sciatherica, Leigh-Sharpe, presented to me by Michael G. L. Perkins, and taken by him from various Gadus morrhua at Lowestoft, in August, 1918 (vide Parasitology, XI, 118, and footnote), was another tube containing about a dozen further specimens taken by him at the same date, from the identical hosts. He first called my attention to the fact that we had two distinct species of Clavella from the same fish, and to him I am primarily indebted for most of the following observations. The claims of the other specimens to be considered as members of a distinct and new species are:—

(1) Body less globose, longer than wide.

(2) Ovisacs tapering.

Breeding in marine animals has long been recognised in a vague way as being in some degree dependent upon sea-temperature. This attitude is well shown in Lo Bianco's last valuable contribution to our knowledge of the breeding periods of practically the whole fauna of the Gulf of Naples. In the Introduction to this work Lo Bianco (1, p. 531) gives tables of temperature observations for a number of years, but does not attempt to correlate them with the breeding periods observed. There are in the literature some observations of reproduction in particular animals being dependent upon particular temperatures, notably by Schmidt (2); but no attempt has been made, so far as I know, to show in a general way the connexion between breeding and temperature in marine animals. It is clear, however, from the writings of Appellöf that this naturalist had anticipated in his mind some of the results obtained from the experimental observations described briefly in the following pages and of the deductions made therefrom. It is believed that these experimental observations have afforded a clue which gives an insight into the way in which breeding may be correlated with temperature in a large number of marine animals, but owing to the inadequacies of breeding records and of observations on physical conditions in the sea only a brief survey has been attempted here, and a fuller discussion of the phenomena is reserved for a later work. In the following pages the extrusion of ripe ova, or the fertilisability of eggs with subsequent development, is taken as the criterion of breeding.