CRUSTACEA 



175 



entirely hidden and well protected ; when submerged in the 

 water, and desiring food, it lifts the four upper valves so that 

 an opening is disclosed centrally, and through this there 

 slowly appears a number of beautiful little curved feathery 

 appendages, which by their lashing movement catch any food 

 particles in the water, driving them down in a current of 

 water into the mouth, which lies hidden within, and which 

 has mandibles and two pairs of soft jaws (maxillae) with 

 whiqh to masticate the food. 



If the animal is startled, the projecting appendages are 

 very rapidly withdrawn, and the shell tightly closed once 

 more. 



To investigate the structure more thoroughly, it is necessary 

 to break away one side of the hard case of a dead specimen, 

 and expose the body and limbs as shown in Fig. 112. Even 

 then the structure is 

 difficult to-understand, 

 and can only be rightly 

 interpreted in the light 

 of a knowledge of its 

 mode of development 

 from the little free- 

 swimming larva that 

 hatches from the egg 

 and gradually changes 

 to the adult form. This 

 larva becomes Cypris- 

 like with a bivalve shell, 

 and then after a time 

 fixes itself, back down- 

 wards, to some object 

 by means of its tiny 

 first antennae (Fig. 112, 

 A). The head below 

 the antennae much enlarges and alters ; it secretes the base 

 of the shell fixing it to a rock; then the mantle of skin 

 round the body secretes the rest of the shell-plates. Finally 

 there results a curiously modified degenerate creature such 

 as is shown in the figure, which lies on its back with its 

 head attached to its shell, and with its mouth placed half- 

 way down the body-mass (Fig. 112), close to the bases 



M.c 



Fig. 112. — The Acorn Barnacle with the right 



half of the shell removed. (After Darwin.) 

 A, Antennae ; 6'(, stomach ; Lh, upper lip, covering 



moutli ; Cr, thoracic legs ; MC, mantlo cavity ; 



D, oviduct. 



