GENERAL CHARACTERS OF CIRRHOPODA. 293 



simple, whilst the two posterior are bifid. Hence, if their 

 early state be alone considered, the Cirrhopods might be regarded 

 as an aberrant order of Crustacea ; but the complete change of 

 form which they exhibit in their passage towards the adult state, 

 involving the development of so many new parts, to which we 

 find nothing analogous in the class of Crustacea, seems to remove 

 them to a greater distance than even the aberrant Lerneida 

 ( 815). 



828. The Class is divided into two principal groups, differing 

 in many of their characters, though agreeing in those which have 

 been hitherto described. 



I. PEDDNCULATA, or LEPADID^:. This Order, including the 

 Lepas or Barnacle and its allies, consists of those Cirrhopods 

 which are attached by a long fleshy peduncle or foot-stalk; 

 their form is flattened at the sides ; and the opening through 

 which the cirrhi are protruded is along one edge. The mantle 

 is sometimes cartilaginous ; but in general it is covered by five 

 calcareous pieces, of which the two principal ones resemble the 

 valves of a Mussel or other Bivalve shell. 



II. SESSILIA, or BALANID^E. In this Order are associated 

 those which attach themselves at once by their basis, like the 

 Balanus, or Acorn-shell, so common on our rocky shores. In 

 these, also, the shell is composed of several valves; but these 

 are united into a cylindrical or somewhat conical tube, which is 

 attached at one end, and open at the other to give passage to the 

 arms, &c. 



ORDER I. PEDUNCULATA. 



829. Of the common Barnacle, and its allies, which together 

 make up this Order, some species are common in nearly all seas. 

 Most of them fix themselves to wood in preference to anything 

 else ; so that a piece of timber, which has been for sometime 

 floating in the ocean, is almost sure to be partly covered by them, 

 and ships 1 bottoms, if not protected by copper, are rendered so 

 foul, as greatly to impede their rate of sailing. This will be 



