238 MOLLUSCA. 



bodies in considerable numbers. They are found, not only on float- 

 ing wood, the hulls of ships, bottles, and other articles floating about, 

 but on shells, on turtles, whales, and even sea-snakes. Large logs of 

 timber are sometimes completely covered with them, compacted in 

 close array, writhing and twisting about, like the serpents on Medusa's 

 head, and presenting a most remarkable spectacle. 



The Acorn-shells (Balani), (Fig. 182), in their general structure 

 and habits resemble the Barnacles ; they have, however, no pedicle 



FlG. 182. ACORN-SHELLS ON THE SHELL OF A WHELK. 



or foot-stalk, but are sessile that is, are fixed immediately to the 

 substances upon which they attach themselves, or in which they are 

 more or less imbedded. The common species is often found in great 

 numbers, covering the shells of mussels and oysters, and may thus 

 be easily procured for examination. Taking one of these as an 

 example of the group, we find them to consist of a shelly cone com- 

 posed of various pieces, accurately fitted together, and capable of 

 enlarging with the growth of the animal. A thin calcareous base 

 or closing plate, fixes the cone to the substance on which it rests, 

 while its apex presents four pieces, that form an operculum or valvu- 

 lar lid, so disposed as to shut up the aperture when the animal 

 retires into its shell ; but easily opened for the protrusion of the 

 cirri. The Acorn-shells are widely spread; groups of different 

 species are found covering rocks, floating wood, shells of various 

 kinds, or even the backs of crabs and lobsters ; any objects, in short, 

 which will afford them a secure resting-place. 



MOLLUSCA. 



Widely different in their appearance from the 

 Articulata described in the last chapters are the 

 creatures that next present themselves. Instead of 

 possessing an external skeleton, divided into numerous 

 segments and furnished with jointed limbs, the 

 Mollusca are either entirely naked and defenceless, 

 or else have their bodies protected by shells, in 



