ANNULOSA: CRUSTACEA. 



28; 



cavity, and is again expelled through a wide orifice." (Fritz 

 Miiller). 



ORDER III. CIRRIPEDIA. Adult attached, enclosed in an 

 integumentary sac, within which a many-valved shell is typicallv 

 developed. Antenna modified for adhesion. Abdomen rudimen- 

 tary. Limbs usually present, in the form of multiarticulate cirri. 

 Sexes generally united. Young locomotive. 



This sub-class includes, amongst others, the common Acorn- 

 shells and the Barnacles or Goose-mussels. All the Cirripedia 

 are distinguished by the fact, that in the adult condition they 

 are permanently fixed to some solid object by the anterior 

 extremity of the greatly metamorphosed head ; the first three 

 cephalic segments being much developed, and enclosing the 

 rest. of the body. The larva is free and locomotive, and the 

 subsequent attachment, and conversion into the fixed adult, is 

 effected by means of a peculiar secretion, or cement, which is 

 discharged through the antennae of the larva, and is produced 

 by a special cement-gland, which is really a portion of the ovary. 

 In the Cirripedia, therefore, the head of the adult is per- 



Fig. 142. Morphology of Cirripedia. A, Lepas pectinata, one of the Barnacles, one 

 side of the shell being removed, enlarged four times : c Peduncle ; d Cement-duct ; 

 o Ovary ; s Ovisac ; v Vas deferens ; p Penis. B, Paecilasma fissa, enlarged five 

 times ; c Peduncle. C, Balanus balanoides, viewed from above, of the natural size. 

 D, Balanus tintinnabulum^ with the shell on one side removed to show the ani- 

 mal : a One of the valves ("scutum") of the operculum; b Another valve ("tergum") 

 of the operculum. (After Darwin and Pagenstecher.) 



manently fixed to some solid object, and the visceral cavity is 

 protected by an articulated calcareous shell, or by a coriaceous 

 envelope. The posterior extremity of the animal is free, and 

 can be protruded at will through the orifice of the shell. This 

 extremity consists of the rudimentary abdomen, and of six pairs 



