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 

 remain as simple, but strong, stumps the mandibles ! 

 Two 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 

 is marked with rings or segments, and behind it follows 

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

 ending with two little hairy prongs like a pitchfork. 

 The right and left movable, shell-like fold, or down- 

 growth, of the sides of the body encloses the whole 



