CRUSTACEA. 



149 



their commijn possession of a regulai- calcareous shell. Two distinct 

 types of structure are known amongst tlie Cirripvili'i.^ constituting 

 the two families of the Barnacles {Li-padidiii)^ and the Acoi-u-shells 

 {flalaniihx). 



In the Barnacles (fig. 99), the anterioi- 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, sucli as a rock, a floating 

 log of timber, or even some marine animal. In tlie Acorn-shells 

 (fig. 100) which occur in myriads ujjon every solitl object between 

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



Fig. 99. — Two fuUy-gniwii individuals of tlic 

 common Barnacle (Lapas aiuUiferu), grow- 

 ing upon a foreign body, p The -stalk of 

 attachment; c The body of the animal en- 

 closed in a shell, from which the legs can 

 be protruded. 



F:g 100. — A, Acorn-shell (Dulamis 

 hOanijldt-i), of the natural size, 

 viewed from above. B, Bcdanus, 

 with the shell on one side removed, 

 to show the animal inside ; a and h 

 Two of the plates fuDiiiug the lid of 

 the shell. 



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

 limpet-shaped or conical shell, comj)Osed of several pieces, and hav- 

 ing an a])erture at its summit. This opening is closed by a movable 

 lid, and from it tlie 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 

 11 



