It is highly interesting to note the manner in which the sea-squirt obtains its food. The nutritive wherewithal consists of sedimentary matters, such as minute animals and plants, these substances being drawn into the breathing-sac along with the currents of water which are continually being taken into the body. The nutritive sediment is collected together by certain folds of the lining membrane of the breathing-chamber, and is thus transferred to the mouth-opening below. The breathing-chambers of the sea-squirts, it may be noted in passing, frequently afford lodgment to tiny dwellers in the shape of little pea-crabs. The writer has noticed these little lodgers to issue forth at night from the mouth of the sea-squirts, when the latter have been kept in an aquarium, in order to pick up particles of food on the floor of their abode. The crabs retreated to their shelter on the slightest alarm; and this case of companionship presents one of those curious instances of animal association which at present we are wholly unable to explain. The food being converted into blood in the digestive system, we may next inquire as to the means which the sea-squirt possesses for circulating the blood through the body. In higher animals, the heart and blood-vessels perform this important task; and in the sea-squirt we find these structures to be represented; the sea-squirt’s heart indeed, in respect of its peculiarity of action, being without a parallel in the whole animal world. The heart consists of a simple tube, from each end of which blood-vessels pass, some being distributed to the breathing-chamber, and others to the body generally. In the highest animals the heart has the double function of driving pure blood through the body, and of circulating impure blood through the breathing organs for purification. It is noteworthy to observe, that by a curious and, as already remarked, altogether unparalleled contrivance, Nature has succeeded in causing the simple tube-heart of the sea-squirt to perform the work done by the complex organ of higher animals. When we observe the movements of the sea-squirt’s heart, we may see it to propel the blood by its pulsations at first to the breathing-chamber for purification. Then a pause succeeds, and the heart is observed to pulsate in the reverse direction, and to drive the blood from the breathing-chamber through the body. Probably no better illustration of the manner in which, by a modification of function, Nature compensates for simplicity of structure, could be had, than that afforded by the sea-squirt’s heart.