STRUCTURE OP CIRRHIPEDA. 
109 
the Barnacle tribe, forming the class Cirrhipeda, 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 of Fig. 50. -Body of 
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 
