20O MANUAL OF ZOOLOGY. 



or larva, into the swollen and deformed adult, must be regarded 

 as an instance of " retrograde metamorphosis." 



SUB-CLASS II. CIRRIPEDIA. 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 metamor- 

 phosed 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 con- 

 version 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 permanently 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 abdomen, 

 and of six pairs of forked, ciliated limbs, which are attached to 

 the thorax, and serve to provide the animal with food. The 

 two more important types of the Cirripedia are the Acorn- 

 shells (Balanidce) and the Barnacles (Lepadidce.) In the former 

 the animal is sessile, the larval antennae, through which the 

 cement exudes, being imbedded in the centre of the mem- 

 branous or calcareous "basis" of the shell. In the latter the 

 animal is stalked, and consists of a " peduncle" and a "capi- 

 tulum." The peduncle consists of the anterior extremity of 

 the body, with the larval antennse, usually cemented to some 

 foreign body. The capitulum is supported upon the peduncle, 

 and consists of a case composed of several calcareous plates, 

 united by a membrane, enclosing the remainder of the animal. 



Before giving a more detailed description of this singular 

 and important sub-class, the following definition, as given by 

 Owen, may be advantageously appended : 



"Body, chitinous, or chitino-testaceous, sub-articulated, mostly symme- 

 trical, with aborted antennae and eyes. Mouth, prominent, composed of a 

 labrum, palpi, two mandibles, and two pairs of maxillae. Thorax, attached 

 to the sternal internal surface of the carapace, with six pairs of multiarti- 

 culate, biramous, setigerous limbs. Abdomen, rudimentary. Vascular 

 system diffused ; white blood. Branchiae, when present, attached to the 

 inferior lateral part of the surface. Most are hermaphrodite ; a few have 

 minute, rudimentary, male individuals, parasitically attached to the females. 

 Penis, proboscidiform, multiarticulate, attached to the hinder end of the 

 abdomen. No oviducts. Metamorphosis and metagenesis, resulting in a 

 permanent parasitic attachment of the fully-developed female or herma- 

 phrodite individual." 



