ANNULOSA I CRUSTACEA. 



2 3 I 



In the symmetrical Sessile Cirripedes or Balanidce, commonly 

 known as Acorn-shells (fig. 86, a), the animal is protected by 

 a calcareous shell, formed by calcifications within the walls 

 of the first three cephalic segments. The animal is placed 

 within the shell, head downwards, and is fixed to the centre of 

 a shelly or membranous plate, which closes the lower aperture 

 of the shell, and which is termed the "basis." The "basis" 

 is fixed by its outer surface to some foreign object, and is 

 sometimes compact, sometimes porous. Above the basis 

 rises a limpet-shaped, conical, or cylindrical shell, which is 

 open at the top, but is capable of being completely closed by a 

 pyramidal lid or " operculum." Both the shell itself and the 

 operculum are composed of calcareous plates usually differing 

 from one another in shape, and distinguished by special names. 

 Within the shell the animal is fixed, head downwards. The 

 thoracic segments, six in number, bear six pairs of limbs, each 



Fig. 86. Morphology of Cirripedia. a Sessile Cirripede or Balanoid, Balanus 

 b Pedunculate Cirripede or Lepadoid, Lepas anatifera. 



of which consists of a jointed protopodite and a much segmented 

 exopodite and endopodite, both of which are ciliated, and con- 

 stitute the so-called "cirri," from which the name of the sub- 

 class is derived. These twenty-four cirri the " glass hand " 

 of the Balanus are in incessant action, being protruded from 

 the opening of the shell, and again retracted within it, con- 

 stantly producing currents of water, and thus bringing food to 

 the animal. There are no specialised respiratory organs in the 

 family of the Balanidtz. Balani sometimes attain a very con- 

 siderable size, and Balanus psittacus is largely eaten on the coast 

 of Chili. 



The remaining family of the Sessile Cirripedes is that, of the 

 VerrucidcE, comprising only the single genus Verruca, In many 

 respects the Verrutidce approach the Balanida, but the shell is 

 composed of six valves only, and is unsymmetrical, whilst the 



