The branchial sac occupies the upper part of the animal; its interior surface is divided into cells by plaits or ridges, which are clothed with cilia; its orifice is surrounded by a circle of tentacles, and its inferior extremity merges into the digestive tube. For as the food of these animals consists of microscopic organisms, which are drawn in by the entering currents, the same influx of water brings oxygen for the respiration, and food for the nutriment of the system.
The water, in yielding its vital properties, passes towards the bottom of the body, and then returning by an upward course is discharged through an orifice closely resembling the other in appearance, and situated near to it on one side. Both orifices can be completely closed at will, by being drawn together in wrinkles, and each is commonly surrounded with minute coloured specks, that are considered to represent eyes. These specks are usually eight around the receiving orifice, and six around the discharging one.