STKUCTUKE OP CIRBHIPEDA. 109 



the Barnacle tribe, forming the class CIBRHIPEDA, or tendril- 

 footed animals. They agree with the lower Mollusca, in 

 being fixed to one spot during all but the earliest period of 

 their lives; the shell being sometimes attached by a long 

 membranous or leathery tube, as that of the Barnacle 

 (fig. 49) ; and sometimes being itself fixed on the surface of 



Fig. 49. SHELL or Fig. 50. -BODY OP 



THE BARNACLE. THE BARNACLE. 



a rock, or on another shell, as is that of the Balanus or 

 acorn-shell. In both cases, the form and structure of the 

 animal are essentially the same. When taken from the shell 

 (in which it lies doubled up, as it were) and spread out, its 

 articulated nature is evidenced by its division into segments, 

 and by the regularity of the arrangement of their tendril-like 

 appendages. These are not formed like legs, since they could 

 be made no use of, the animal being incapable of moving 

 from place to place ; but they serve to produce currents in 

 the surrounding water, by which food is brought to the 

 mouth, and the blood is submitted to the influence of a 

 fresh supply of air. The nervous system of this group 

 is formed precisely upon the plan of that of the Articulata 

 generally ( 94) : and if any doubt could have remained as to 

 its true place in the series, it is removed by the knowledge 

 of the fact, that the animals composing it bear a strong 

 resemblance in their early condition to some of the lower 

 Crustacea, possessing eyes and legs, and swimming freely 

 about; and that they attain their adult form by passing 



