METAMOEPHOSES. 459 



These leaves appear to be composed of a layer of ova, irregularly placed, 

 and imbedded in a kind of parenchymatous texture, out of which they 

 readily fall, when about to hatch, on its substance being torn asunder ; 

 indeed, it appears at length to become so tender as to fall entirely away, 

 so that, after the period of gestation is passed, no vestige of these leafy 

 conceptacles is to be found." 



(1184.) In the second form of CIREHOPODA (Balani), the animals, 

 instead of being appended to foreign substances by elastic and flexible 

 pedicles, are sessile, the shelly investment of the body being in imme- 

 diate contact with the rock or other submarine body to which the 

 Barnacle adheres. The soft tube of Pentelasmis is, in this case, repre- 

 sented by a strong testaceous cone composed of various pieces accurately 

 joined together, and generally closed inferiorly by a calcareous plate ; 

 while the representatives of the valves of the pedunculated species 

 form a singular operculum, which is moved by special muscles, and 

 accurately shuts the entrance of the shell when the animal retires into 

 its abode. In their general structure, however, the Balaniform Cirrho- 

 pods accord with the description above-given ; and, from the similarity 

 of their habits and economy, a more elaborate account of the peculi- 

 arities which they exhibit would be superfluous in this place. 



(1185.) One of the most remarkable circumstances connected with 

 the history of the CIKRHOPODA 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 other- 

 wise, the newly-hatched young are endowed with locomotive 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. Y. Thomp- 

 son, 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 Balanvs pusillus ; and the following is that gentleman's account 

 of their appearance and subsequent change. The young Cirrhopod is a 

 small translucent animal, r \jth of an inch long, of a somewhat elliptic 

 form, but very slightly compressed laterally, and of a brownish 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 provided with a cup-like sucker and hooks, serving 

 * Zoological Researches, 4th Memoir, 1830. 



