

ANNULOSA: CRUSTACEA. 179 



terior 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 

 (Lepadidoe), In the former the animal is sessile, the larval 

 antennae, through which the cement exudes, being imbedded 

 in the centre of the membranous or calcareous ' basis ' of the 

 shell. In the latter the animal is stalked, and consists of a 

 ' peduncle ' and a ' capitulum.' The peduncle consists of the 

 anterior extremity of the body, with the larval antennae, 

 usually cemented to some foreign body. The capitulum is sup- 

 ported 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 multi- 

 articulate, 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. ISo oviducts. Metamorphosis and metagenesis, resulting in a 

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

 phrodite individual.' 



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

 at first oval in shape, devoid of segmentation, and protected 

 by a dorsal carapace. There is a single eye, two pairs of 

 antennae, and three pairs of natatory limbs, of which the two 

 posterior pairs are bifid at their extremities. The larya, in 

 met, in this stage very closely resembles some of the smaller 

 Entomostraca. In a more advanced stage, the carapace be- 

 comes considerably altered, the great antennae become modi- 

 fied for prehension, the first pair of antenna? (antennules) 

 disappears, and the eye becomes double. In a still more 

 advanced condition, the anterior pair of natatory limbs become 

 bifid, like the posterior ones ; three additional pairs of limbs 

 are developed behind those already existing ; the abdomen be- 

 comes defined from the thorax ; and the carapace is composed 

 of two portions, or valves, which enclose the thorax and abdo- 

 men with their appendages. Finally, the young Cirripede fixes 



N 2 



