BARNACLES AND OTHER CRUSTACEANS in 



which one can imagine to be hinged along a line running 

 down the back so as to open like the covers of a book. 

 There are very common little, free-swimming " water- 

 fleas " (minute crustaceans) of many hundreds of kinds 

 which have hinged shells of this description when in the 

 full-grown condition, and it is found that the young 

 barnacles and sea-acorns pass through a free-swimming 

 phase of growth (the Cyprid stage), in which they greatly 

 resemble these " water-fleas." 



In fact, it is quite easy to hatch the young from the 



eggs of either ship's barnacles or acorn-barnacles at the 



right season of the year. They commence life as do so 



many Crustacea — in the " nauplius state," with three pairs 



of jerking limbs (Fig. 9). As they grow the overhanging 



pair of shells, delicate and transparent, appear ; the three 



pairs of nauplius legs lose their swimming power ; the 



most anterior (always called antennules in all crustaceans) 



become elongated and provided each with an adhesive 



sucker, on the face of which a large cement gland opens, 



secreting abundant adhesive cement ; the second pair 



(antennae) shrivel and disappear altogether ; the third 



pair lose their long blades for striking the water and 



amain as simple, but strong, stumps — the mandibles ! 



wo new pairs of little jaw-feet appear behind these, 



and farther back on the now enlarged body (the whole 



creature is not bigger than a small canary seed !) six 



pairs of Y-shaped legs appear and strike the water 



rhythmically, so that the little creature swims with some 



sobriety. The region to which these legs are attached 



s marked with rings or segments, and behind it follows 



I small, limbless, hind body of four segments, or joints, 



ending with two little hairy prongs like a pitchfork. 



|fhe right and left movable, shell-like fold, or down- 



jjrowth, of the sides of the body encloses the whole 



