1. This paper records observations upon living specimens only of the Choanoflagellate Codosiga botrytis, Ehrbg.
2. The study of the organism does not support the view put forward by Entz, Franzé, and Ehrlich, that the collar is a spiral membrane.
3. It shows that the collar of Codosiga botrytis is, as earlier workers correctly stated, a protoplasmic, flexible, retractile, conical membrane, closed on all sides except on its upper, free surface. Its function as an apparatus for catching food is discussed.
4. The flagellum and the currents it produces are described, and the part played by them in bringing food to the collar, and their other functions, are explained.
5. It is shown that the food, when it has been caught by the collar, passes down the outside of that structure to be ingested at or below the middle of the body and not at its upper end and never by the area enclosed by the collar. Defoecation, however, invariably occurs from the area enclosed by the collar.
6. The actual ingestion of food is effected by means of a rhythmical contraction which passes round the upper end of the body just below the collar, acting in conjunction with the mucous envelope in which the body is enclosed. As this wave of contraction passes ronnd it creates a space between the body and the envelope at successive points along its course. It is this space which, being seen only in profile at the sides of the body, has been interpreted either as a travelling vacuole (Bütschli) or as a portion of a spiral membrane (Entz, Franzé, Ehrlich). It is into this space that the food particle drops when it has reached the base of the collar. It is then sealed up in it, so to speak, as the wave of contraction passes on, by the return of the body-surface to close apposition to the envelope above it. Being now interposed between the body and the mucous envelope, it appears to lie in a vacuole in this situation ; and it is eventually pressed into the interior of the body when, by a continuance of the amoeboid movement, the body again moves into close contact with the mucous envelope over the area occupied by this apparent vacuole.
7. There are normally two contractile vacuoles situated at different levels and on opposite sides of the posterior half of the body. Entz and his school are wrong in interpreting one of these as a so-called ‘gullet vacuole’ at which food is said to be ingested.
8. No individuals have as yet been seen that show either division, encystment, or conjugation.