We caught this day a number of Velella, which are furnished with crests; some of them were dead, and nearly always when such was the case we found a species of barnacle attached in great numbers to them. When these animals had only recently died, so that the whole of their blue base had not been detached from them, the barnacles were generally very minute, so that the naked eye could only just detect them, and there were no large barnacles on the same fish: now, how did the minute ones get there? As the barnacles grew larger, the remains of the velella changed into large excrescences, half the size of a walnut. We caught also several little animals, all of the same species, which swam about on the surface of the water with the greatest rapidity, performing the same kind of evolutions that we see in a little black and white insect (Gyrinus) which swims on the top of tranquil pools in England. July 16. This day a curious animal was caught, perfectly diaphanous; total length 0.8 inch; length of third leg, 0.4 inch; this was provided with a claw like a crab; head shaped like a grasshopper, 0.2 inch in length, and placed like the head of a grasshopper, at right angles to the body; eyes black and prominent, apparently four, two on each side; first and second legs of nearly the same length; the third leg nearly double the length of either of the others; five on each side. The top of the head is divided into two prominent knobs, one on each side, which, viewed through a microscope, appear to be minutely reticulated. The animal may be considered as consisting of four portions: the head; the upper part of the body, 0.18 inch in length, and divided into five rings; the lower part, consisting of one shield-like portion, 0.12 inch in length, the body at the lower portions of this decreases almost to the thickness of a thread; the tail, 0.3 inch in length, and divided into three shield-like pieces, laid one over the other as in the shrimp (imbricated); at the lower extremity of each of these scales there is on each side a fin-like leg, in addition to those above-mentioned. Breadth of the animal across its head, 0.2 inch, and this was the broadest part of it. It lived for some time out of water, and even when put into spirits, it swam in an extraordinary manner, falling head over heels every time, which motion it accomplished by swimming on its back and making rapid strokes with the fin-like legs with which it is provided behind. We also caught today several little crabs and barnacles. I kept one specimen, to show old and young barnacles attached to the same Velella.