ANNULOSA: CRUSTACEA. 211 



free and locomotive, and the subsequent attachment, and con- 

 version into the fixed adult, is effected by means of a peculiar 

 secretion, orcement, which is discharged through the antennas 

 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 (BalanidcB) and the Barnacles (Lepadidce). In the former 

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

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

 ous 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 antennae usually cemented to some 

 foreign body. The capitulum is supported upon the peduncle, 

 and consists of a casecomposed of several calcareous plates, 

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

 Before giving a more detailed description of this singular 

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

 Owen, may be advantagedusly appended : — • 



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

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

 labrum, palpi, two mandibles, and two pairs of maxillse. 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 fuUy-developfed female or herma- 

 phrodite individual." 



As regards the development of the Cirripedia, the larva is at 

 first a " Naupli.us," with an unsegmented pyrifonn body, a 

 median eye, and a dorsal shield or buckler. The abdomen is 

 produced beneath the anus into a long forked caudal appen- 

 age, and there is a long spine over the amis. A mouth, intes- 

 tine, and vent are present. After several moults the young 

 NaupHi become " pupas " (fig. 64). The dorsal shield is 

 folded so as to form a bivalve shell ; the anterior limbs (anten-; 



