The fame of the sea-squirts is by no means of modern date. Aristotle gives us a succinct description of them in his _History of Animals_ under the designation ‘Tethea;’ and by the same name Pliny has made the sea-squirts of classic reputation, since we learn from this latter author that they were included as articles of importance in the pharmacopœia of the Romans. In their commonest phases, the sea-squirts appear as little leathery bags of clear aspect, through the somewhat transparent wall of which the internal organs can be discerned. The resemblance of the animals to the ancient wine-skin has already been remarked. The wine-skin, as every one knows, was made of the stomach of some animal, or of the skin so arranged as to present two orifices or necks. Thus when we look at a common sea-squirt we see a veritable little ‘leather-bottel,’ measuring from half an inch to an inch or more in length, attached by one extremity to the rock at low-water mark, or to the shell we have dredged, and bearing on its upper surface two prominent openings, each supported on a short neck. The origin of the common name of ‘sea-squirt’ is by no means hard to trace. The incautious observer who picks up a sea-squirt which has through unpropitious fate been cast up on the sea-beach after a storm, after a short survey of the sac-like body, may possibly be tempted to squeeze it as a preliminary to further investigation. On being thus irritated, the animal will most likely retaliate by forcibly ejecting jets of water from the two orifices of the ‘bottel;’ this procedure possibly resulting in the relinquishment of the sea-squirt as altogether an unlikely and unfavourable object for further study. But the observation of this unpolite habit on the part of the animal, will be found to assist our further comprehension of its physiology, and of the manner in which the functions of its life are carried on.