CIRRHOPODA. 353 



ing so insensibly from one to another, that the precise boundary- 

 line which separates them is not always easily defined. The CIR- 

 RHOPODS, or Barnacles, upon the consideration of which we are 

 now entering, present a remarkable exemplification of this import- 

 ant fact ; and are found to be so strictly intermediate, both in 

 external configuration, and even in their anatomical construction, 

 between the HOMOGANGLIATA, which have recently occupied 

 our attention, and the great class of beings that next presents itself 

 for investigation, that these animals might, with almost equal pro- 

 priety, be located either among the Articulated or Molluscous 

 tribes of Invertebrata ; and it will not be surprising, if, after read- 

 ing the details connected with their structure, some naturalists 

 should prefer to regard them as belonging to the former rather 

 than to the latter division. The CIRRHOPODA, indeed, present a 

 strange combination of articulated limbs, united with many of the 

 external characters of a Mollusk, as will be at once evident from 

 the examination of any species of Barnacle whether sessile or pe- 

 dunculated. We select a common form, Pentalasmis vitrea, as 

 an example of the kind last mentioned. The animal in question 

 is enclosed in a shell resembling in some respects that of the com- 

 mon mussel, but composed of five distinct pieces, united together 

 by a dense intervening membrane : of these, four pieces are lateral, 

 and disposed in pairs ; while a fifth, which is single, is interposed 

 between the posterior edges of the two valves, so as to unite them 

 along the whole length of the back. Along the anterior margin 

 the valves are only partially connected by membrane, so that a long 

 fissure is left through which the articulated extremities may be 

 protruded. In place of the hinge that joins the two shells of the 

 mussel, we find the tough coriaceous membrane that unites the dif- 

 ferent shelly pieces of the integument of Pentalasmis, prolonged 

 into a cylindrical pedicle (Jig. 169, /), which is in some species 

 many inches in length, and, being attached by its extremity to any 

 submarine body, fixes the animal permanently to the same locality. 

 The external layer of this pedicle is coriaceous or almost corneous 

 in its appearance, being evidently an epidermic structure ; but, in- 

 ternally, the tube is lined with a layer of strong muscular fibres 

 arranged longitudinally (Jig. 169, m, n), which, by their contrac- 

 tion, are no doubt able to bend the flexible stem in any given 

 direction, and thus confer upon the animal a limited power of 

 changing its position when necessary. On removing one half of 

 the shelly covering, as in Jig. 167, <z, a, we expose the body of the 



