ENTOMOSTRA CA. 



2(st) 



Balamis, the acorn -shell, encrusts 

 between high and low water 

 marks. It may be described, 

 in Huxley's graphic words, as a 

 crustacean fixed by its head, 

 and kicking the food into its 

 mouth with its legs. The body 

 is surrounded, as in Lepas, by 

 a fold of skin, which forms a 

 rampart of six or more calcar- 

 eous plates, and a fourfold lid, 

 consisting of two scuta and two 

 terga. When covered by the 

 tide, the animal protrudes and 

 retracts between the valves of 

 the shell six pairs of curl-like 

 thoracic legs. The structure 

 of the acorn-shell is in the main 

 like that of the barnacle, but 

 there is no stalk. 



The life history also is similar. 

 A Nauplius is hatched. It 

 has the usual three pairs of 

 legs, an unpaired eye, and a 

 delicate dorsal shield. It moults 

 several times, grows larger, and 

 acquires a firmer shield, a 

 longer spined tail, and stronger 

 legs. Then it passes into a 

 Cypris stage, with two side 

 eyes, six pairs of swimming legs, 

 a bivalve shell, and other 

 organs. As it exerts itself 

 much but does not feed, it is 

 not unnatural that it should 

 sink down as if in fatigue. It 

 fixes itself by its head and 

 antennx, and is glued by the 

 secretion of the cement gland. 

 Some of the structures, e.g. the 

 bivalve shell, are lost ; new 

 structures appear, e.g. the 

 characteristic Cirriped legs and 

 the shell. Throughout this 

 period, which Darwin called 

 the "pupa stage," there is 

 external quiescence, and the 

 young creature continues to 

 fast. The skin of the pupa 

 moults off; the adult structures 

 and habits are gradually assumed 



the rocks in arcal numbers 



Fig. 115. — Development of Sacculina. 



— After Delage. (Not drawn to 



scale. ) 

 A, Free-swimming Nauplius, with three 



pairs of appendages ; 6, pupa stage ; C, 



adult protruding from the abdomen of a 



crab. 



At frequent periods of continued 



