CIRRHOPODA. 361 



entrance of the shell when the animal retires into its abode. In 

 their general structure, however, the Balaniform Cirrhopods accord 

 with the description above given ; and, from the similarity of their 

 habits and economy, a more elaborate account of the peculiari- 

 ties which they exhibit would be superfluous in this place. 



(397.) One of the most remarkable circumstances connected with 

 the history of the CIRRHOPODA, is the recently discovered fact of 

 their undergoing a distinct metamorphosis ; so that, in the earliest 

 periods of their existence, instead of being rooted by means of a 

 pedicle or otherwise, the newly hatched young are endowed with lo- 

 comotive organs, calculated to enable them to swim freely about, and 

 giving them rather the appearance of Entomostracous Crustacea, 

 than of animals of their own class. This singular fact was first 

 announced by Mr. J. V. Thompson, of Cork ;* and its correctness 

 has since been admitted by various anatomists who have devoted 

 their attention to this subject. Mr. Thompson's first observations 

 were made upon minute animals, which, although at first actually 

 taken for Crustaceans, turned out to be the young fry of Balanus 

 pusillus ; and the following is that gentleman's account of their 

 appearance and subsequent change. The young Cirrhopod is a 

 small translucent animal one-tenth of an inch long, of a somewhat 

 elliptic form, but very slightly compressed laterally, and of a brown- 

 ish tint. When in a state of 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 of the animal are withdrawn within the shell, which appears to be 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 limbs. The limbs are of 

 two descriptions : viz. anteriorly, a large and very strong pair pro- 

 vided with a cup-like sucker and hooks, serving solely to attach 

 the animal to rocks, stones, &c. ; and posteriorly, six pairs of na- 

 tatory members, so articulated as to act in concert, and to give a 

 very forcible stroke to the water, causing the animal, when swim- 

 ming, to advance by a succession of bounds after the same manner 

 as the water-flea (Daphnia) and other Monoculi, but particularly 

 Cyclops, whose swimming-feet are extremely analogous. The 

 tail, which is usually bent up under the belly, is short, composed 

 of two joints, and terminates in four setae, forming an instrument 

 of progression. The animal, moreover, is furnished with large 

 * Zoological Researches, 4th Memoir, 1830. 



