280 



LECTUKB XIII. 



the calcified and uncalcified cliitine. In the growth of the body the 

 membrane between the valves splits periodically, and a new strip of 

 membrane is formed beneath, connected on each side with a fresh 

 layer of shell, the old strips disintegrating and disappearing*, but the 

 valves are not shed. The integument of the peduncle consists, in 

 LepaSf of corium, covered by smooth layers of chitine (Jig. 121, A); 

 but in Scalpellum and Pollicipes its outer surface is roughened by 

 small calcified scales. If the peduncle be very carefully removed 

 from the surface of attachment, the larval antennae can always be 

 found quite close to the end, but not at the actual apex.f 



The movements of the peduncle in such species are effected by a 

 strong muscular tunic, consisting of three layers of non-striated fibres, 

 longitudinal, transverse, and oblique (ib. b). In Otion they extend 

 half-way down the sheath of the body J ; and in Alepas they line the 

 whole of the body-sheath. The action of these muscles is antagonised 

 by the elasticity of the chitinous tunic. The common Barnacle 

 approximates its scuta by a strong transverse adductor muscle ; its 

 body or visceral mass is moved towards the aperture of the shell, 

 which is thereby at the same time widened, by longitudinal muscular 

 fibres, and is retracted by shorter fibres attached to its base. In 

 Lithotrya there are two fan-like transvex'se muscles extending from 

 the basal points of the terga to a central line on the under side of 

 the carina. § The cirrigerous arms in all the Cirripeds have powerful 

 muscles for their actions, which are of the utmost importance to the 

 animal, inasmuch as the food is obtained by the currents which they 

 123 produce, and almost incessantly maintain, in 



the surrounding water. The sessile Barna- 

 cles are provided with a series of muscles 

 attached to the margin of the conical shell, 

 which act on the opercular calcareous pieces, 

 and close the opening of the shell. 



The nervous system {fig. 123), first de- 

 scribed and figured by Cuvier in the Lepas 

 anatifera, presents the homogangliate type. 

 The oesophagus is surrounded by a wide 

 oral ring, closed above by a pair of super- 

 oesophageal ganglions, which send off" nerves 

 to the peduncle, the ovaria, and to the 

 aborted and confluent eyes. Mr. Darwin 

 has detected a small ganglion on each op- 

 tic nerve in the Lepas fascicularis.\\ 



* CCXXIII. p. 30. t Il>- P- 33. t Prep- Nos. 62, 68, 69. 



§ CCXXIIL p. 33, II Ib. p. 49. 



Kervous SyBtem. Lepas. 



