CEUSTAOBA. 



137 



shells, both extremely unlike Crustaceans to look at. All the 

 Cirripedes are distinguished by the fact that, while they are 

 quite free when young, and very similar to some of the little 

 Crustaceans just described, when adult they are immovably 

 fixed by their heads to some solid object. In this fixed con- 

 dition the body and internal organs are, in most cases, pro- 

 tected by means of a calcareous shell, composed of many pieces, 

 and the only part of the body which remains movable is the 

 legs, which are constantly thrust out of the shell and again 

 drawn in in quest of food. The Cirripedia were formerly de- 

 scribed as " multivalve " shell-fish {MoUusca), awing to their 

 possession of a regular calcareous shell. Two distinct types 

 of structure are known among the Cirripedia (Fig. 57), con- 

 stituting the two families of the barnacles (Lepadidce), and 

 the acorn-shells (Halanidce). 



In the barnacles (Fig. 57, b), the anterior end of the body 



Fig. 57.— Cirripedia. a Sessile Cirripode (Balanus) ; 5 Stalted Cirripede {Zepm). 



is much elongated, and is converted into a kind of stalk, by 

 means of which the animal is attached to some solid object, 

 such as a rock, a floating log of timber, or even some marine 

 animal. In the acorn-shells (Fig. 57, a), which occur in 

 myriads upon every solid object between tide-marks, there is 

 no stalk, but the head is firmly cemented to the centre of a 

 membranous or shelly plate. The body is enclosed in a limpet- 

 shaped or conical shell, composed of several pieces, and having 

 an aperture at its summit. This opening is closed by a mov- 

 able lid, and from it the animal can protrude its delicate legs 

 or " cirri," which look like a " glass hand," and are constantly 

 employed in sweeping the water in search of food. 



In accordance with the fixed condition of the adult, almost 

 all the Cirripedia are hermaphrodite, possessing both male 



