CRUSTACEA. 147 
in a shell, which is made up of two lateral halves or valves. 
The valves of the shell are united by a membrane along the 
back, but can be opened below, so as to allow of the protrusion 
of the feet. 
ORDER CIRRIPEDIA.—The last order of Crustacea which 
requires mention is that of the C7zrripedia (Lat. cerrus, a curl; 
and #es, foot), comprising the so-called barnacles and acorn- 
shells, both extremely unlike Crustaceans to look at. All the 
Cirripedes are distinguished by the fact that, whilst 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 condi- 
tion the body and internal organs are, in most cases, protected 
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 Czrrzfedia were formerly described as 
“multivalve” shell-fish (AZollusca), owing to their possession 
of a regular calcareous shell. Two distinct types of structure 
are known amongst the Cirrifedia (fig. 70), constituting the 
two families of the barnacles (Lefadzde@), and the acorn-shells 
(Balanide). 
In the barnacles (fig. 70, 4), the anterior end of the body is 
Fig. 7o.—Cirripedia. a@ Sessile Cirripede (Balanus); & Stalked Cirripede (Leas) 
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. 70, 2) which occur in myriads upon every 
solid object between tide-marks, there is no stalk, but the head 
