CEUSTACEA. 



149 



their common possession of a regular calcareous shell. Two distinct 

 types of structure are known amongst the Cirripedia, constituting 

 the two families of the Barnacles {Lepadidce), and the Acorn-shells 

 {Balanidm). 



In the Barnacles (fig. 99), the anterior end of the body 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-shella 

 (fig. 100) which occur in myriads upon every solid object between 

 tide-marks, there is no stalk, but the head is firmly cemented to the 



Fig. 99. — Two fully-grown individuals of the 

 common Barnacle (Xepas aTmtifera), grow- 

 ing upon a foreign 1)047. p Tlie stalk of 

 attachment ; c The body of the animal en- 

 clo-sed in a shell, from which the legs can 

 he protruded. 



Fig. 100. — A, Acorn-sliell (Balanus 

 ialanoides), of the natmal size, 

 viewed from ahove. B, Balanus, 

 with the shell on one side removed, 

 to show the animal inside ; a and 6 

 Two of the plates forming the lid of 

 the shell. 



centre of a membranous or shelly plate. The body is enclosed in a 

 limpet-shaped or conical shell, composed of several pieces, and hav- 

 ing an aperture at its summit. This opening is closed by a movable 

 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 and female 

 organs of reproduction. In some cases, however, males exist, but 



