The investigations in the American Tropics described in this paper were made during the summers of 1911 and 1912 under the auspices of the Carnegie Institution of Washington: the work was made possible by the invaluable help and kindness of Dr. Alfred G. Mayer, Director of the Department of Marine Biology. The investigations in Temperate waters were made from the Plymouth Laboratory of the Marine Biological Association of the United Kingdom, and my thanks are due to the Council and Director of the Association for giving me every facility for the work.

In the past, deep-sea water samples for bacteriological purposes have been taken either by means of water-bottles provided with taps, and so made that the sample came in contact with metal, or in evacuated sterilized glass vessels drawn out to a fine point and sealed in the flame. The water-bottle is difficult to sterilize on account of the taps, and the results obtained are vitiated by the bactericidal action of metals. The sealed glass tube is free from these defects, but at even moderate depths it is liable to collapse on the sudden change of pressure when the end of the capillary portion is broken off.

My thanks are due to Dr. Allen for permitting me to examine this collection.

In all 292 specimens were obtained, but of these 16 were young immature Asteroidea and Echinoidea. It is impossible to identify these with any degree of certainty, and I have not hazarded an opinion about them.

The remainder belong to 17 genera and 24 species. The Ophiuroidea were particularly numerous, 190 specimens belonging to that class.

The classification followed is that of Professor MacBride in the Cambridge Natural History.

The Gammarus here described is found in Chelson Meadow, a large tract of reclaimed land lying near the mouth of the Plym, and protected from the tidal water by an embankment. The land is drained by ditches, which empty into the Plym at low water by means of sluice-gates. The sea-water enters in volume only at the highest tides over the top of the sluice-gates, but there is a constant slight infiltration of salt water through the embankment.

The present paper is the outcome of an investigation made on a species of Gammarus inhabiting both fresh and brackish water (4, p. 657). As this species (G. zaddachi) seemed to show a marked variation in appearance according to its environment, we instituted experiments in the Laboratory at Plymouth on G. chevreuxi to see if a similar variation in appearance could be brought about by varying the degree of salinity of the water in which the animals lived.

No very definite statement has yet been made as to the time when Mytilus edulis spawns at Plymouth. However, in 1911 records of spawning in the Laboratory tanks were made in January, February, and March, and in 1912 two specimens removed from the tanks spawned in early May.

During the summer of 1911 I conducted at the Plymouth Biological Station a series of experiments on the living active gas-gland associated with the bladder of certain marine fish, the results of which are recorded in a paper published in the Anatomischer Anzeiger for 1911 (Bd. XL, p. 225). Whilst so employed I incidentally made some observations on the structure and mode of action of the “oval” in fishes, and since my conclusions differ in several particulars from those of Nusbaum and Eeis (Bull. Acad. d. Sciences Gracovie, 1905, p. 778; Anatomischer Anzeiger, Bd. XXXI, 1907, p. 169), I think it as well to put them on record. Most of my observations were made on the Pollack. If a weight be attached to this fish so as to cause the gas-gland (oxygen gland) to become active and to pump oxygen into the bladder, it will be found that the oval strongly contracts, so as to prevent the additional gas forced into the bladder from escaping into the blood. The oval, it may be mentioned, is a large oval area usually situated in the dorsal posterior wall of the bladder. It differs from the rest of the bladder wall in that it alone is permeable to the contained gases, and, like the ductus pneumaticus in Physostomi, permits their escape when the conditions require it. In the Pollack the oval is normally widely open and is invisible to the naked eye, but on the gas-gland being caused to become active in an unusual degree, the oval becomes strongly contracted and is then a very conspicuous structure inside the bladder. This contraction of the oval is of course effected by muscles, and the result of it is to cause the thin-walled permeable area to become more or less completely shut off from the general bladder cavity, the walls of which, as just mentioned, are impermeable.

Some fifty years ago a German investigator, Eeissner, discovered lying freely in the central canal of the spinal cord of the lamprey a very fine cylindrical rod, which he supposed, notwithstanding its unusual situation, to be a delicate nerve fibre. He failed, however, to learn anything concerning its connection with the central nervous system, and his discovery, although confirmed, seems to have attracted but little attention. The few observers who have since that time recorded observations upon Eeissner's fibre were almost all agreed that it was to be looked upon merely as an artifact produced by the coagulation of the cerebro-spinal fluid by the action of the fixing reagents employed.