284 CIRRHOPODA; BAI.ASUS AND ITS ALLIES. 



that a piece pf timber, which has been for some time floating in 

 the ocean, is almost sure to be partly covered by them, and 

 ships' bottoms, if not protected by copper, are rendered so foul, 

 as greatly to impede their rate of sailing. This will be easily 

 understood, when it is borne in mind how greatly the friction 

 will be increased, when the water, instead of being ploughed by 

 a smooth surface, is held (so to speak) by excrescences, such as 

 those represented in Fig. 615. Many of the species also attach 

 themselves to marine animals, such as Crustacea, slow-moving 

 Fishes, Sea Snakes, Whales, &c. The commonest species, Pen- 

 talasmis anatifera, is the one respecting which so many absurd 

 stories were formerly told (Vol. I. p. 16). The peduncle some- 

 times grows to the length of two or three feet ; and it possesses 

 a considerable degree of contractility, enabling the animal, by 

 its means, to change its place, in some degree, by the shortening, 

 extension, or bending of the footstalk. A large log of timber 

 covered with these animals, twisting and diverging in all direc- 

 tions, and so close as to hide the surface of the log, is a curious 

 sight, looking like an enormous collection of Serpents. In the 

 Conchoderma, the mantle has somewhat of a cartilaginous tex- 

 ture ; and the valves cover only a small portion of the surface. 

 The Lithotryce and Alcippe burrow into rocks and shells ; the 

 latter is a native of the British shores. 



897. In the BALANID^E, or SESSILIA, commonly known as 

 "Acorn-shells," which are met with almost everywhere on our 

 shores, the shell is formed on a very different plan. It consists 

 of six principal valves, of a triangular shape, the base of each 

 triangle being at the bottom, and the apex or 

 point at the top. Within these, however, is an- 

 other set of six smaller triangular valves, ar- 

 ranged in just the opposite direction, so as to 

 fill. up the space between the preceding, and to 

 give to the whole shell somewhat the form of a 

 cone with the top cut off. The aperture of 

 the shell is closed by an operculum, or lid, composed of two or 

 four pieces ; this the animal can apply to the orifice, in such a 

 manner as to close it completely, when its body is entirely drawn 



