CIRRHOPODA. 449 



CHAPTER XVI. 



CIRRHOPODA. 



(1164.) THE CIRKHOPODA 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 pedunculated. We select a common form, Pentelasmis 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 common 

 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 pos- 

 terior 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 cori- 

 aceous membrane that unites the different shelly pieces of the integu- 

 ment of Pentelasmis prolonged into a cylindrical pedicle (fig. 235, Z), 

 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 struc- 

 ture ; but internally the tube is lined with a layer of strong muscular 

 fibres arranged longitudinally (fig. 235, ra, ri), which by their con- 

 traction are no doubt able to bend the flexible stem in any given di- 

 rection, 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 fig. 232, a a), we expose the body of the Cirrhopod, and 

 discern the following particulars. The lower portion of the body, which 

 encloses the principal viscera (b 6), is soft and much dilated, especially 

 towards the dorsal region ; this part of the animal is covered with a 

 delicate membrane, beneath which is a layer of whitish granular sub- 

 stance. The mouth (g) is seen upon the ventral aspect, situated im- 

 mediately at the inferior extremity of that longitudinal fissure in the 

 mantle through which the arms are protruded : the oral aperture 

 appears to be raised upon a prominent tubercle, and, when attentively 

 examined, is found to be provided with a rudimentary apparatus of 

 jaws, presenting a distinct lip, furnished with minute palpi, and three 

 pairs of mandibles, of which the two external are horny and serrated, 

 while the third remains permanently soft and membranous. Imme- 



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