The Acorn-Shell, ([Balanus].)
Mr. Thomson describes his discovery of the real nature of the Cirrhipeds in these words. “On April 28, 1823, which the author had devoted to the investigation of some marine productions, he was returning home without any addition to his stock of knowledge, when, casually throwing out a small muslin towing-net, on crossing the ferry at Passage, such a capture of marine animals was made, as furnished a treat which few can ever expect to meet, and could hardly be excelled for the variety, rarity, and interesting nature of the animals taken.” After mentioning the names of several very rare species, he continues “and others perfectly nondescript, and incapable of being associated in any of our classifications of the Crustacea; of this description is the little animal about to be described.
Balanus, (The Acorn-Shell Barnacle.)
“There is a small translucent animal one-tenth of an inch long, of a somewhat elliptic form, moderately compressed, and of a brownish hue. When in a state of perfect repose, it resembles a very minute mussel, and lies upon one of its sides at the bottom of the vessel of sea-water in which it is placed. At this time all the members of the animal are withdrawn within the shell, which appears to be composed of two valves, united by a hinge along the upper part of the back, and capable of opening from one end to the other along the front, to give occasional exit to the legs. The limbs are of two descriptions; namely, in front a large and very strong pair, provided with a cup-like sucker and hooks, &c., and at the hinder part of the body, six pair of swimming-members, so articulated as to act in concert, and to give a very forcible stroke to the water, so as to cause the animal, when swimming, to advance by a succession of bounds, after the manner of the water flea.
- Fig. 1. Larva of Barnacle, natural size.
- 2. " " "magnified, seen from above.
- 3. " " "highly magnified, seen from side;
- a, swimming-members;
- b, front limb, with sucker.
- 4. Eye, much magnified.
- 5. Perfect Young Barnacle, natural size.
- 6. """ full grown.
“The greatest peculiarity, however, in the structure of this animal, is the eyes; which, although constantly shielded by the valves of the shell, are placed on footstalks, as in the crab and lobster, in front, at the sides of the body.
“Some of these curious creatures were collected in the Spring of 1826; and, in order to see what changes they might undergo, were kept in a glass vessel, covered by such a depth of sea-water, that they could be examined at any time by means of a common magnifying glass; they were taken on May the 1st, and on the night of the 8th, the author had the satisfaction to find that two of them had thrown off their exuviæ[4], and, wonderful to say, were firmly adhering to the bottom of the vessel, and changed into young Barnacles! such as are usually seen intermixed with grown specimens, on rocks and stones, at this season of the year. The eyes were still perceptible, although the principal part of the black colouring-matter appeared to have been thrown off with the exuviæ. On the 10th, another individual was seen in the act of throwing off its shell, and attaching itself, like the others, to the bottom of the glass.”