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.
[Illustration: _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withdrawn within the shell, which appears to be composed of two valves,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.
[Illustration:
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.”