ENTOMOSTRACA. 



289 



valves, nor the uniting membranes, nor the envelope of the stalk, are 

 moulted, though disintegrated 

 portions may be removed in 

 flakes and renewed by fresh 

 formations. In the allied genus 

 Scalpelluin, some are like Lepas, 

 hermaphrodites, without comple- 

 mentary males (Sc. balanoides] ; 

 others are hermaphrodite, with 

 complementary males (Sc. villo- 

 siir/i] ; and others are unisexual, 

 but the males are minute and 

 parasitic (Sc. regiutti). 



Balamts, the acorn-shell, en- 

 cm -ts the rocks in great numbers 

 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 calcare- 

 ous 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 antennne, and 



B 



FIG. 138. Development of Saccii Una. 

 After Delage. (Not drawn to 

 scale. ) 



A, Free-swimming Nauplius, with three 

 pairs of appendages ; B, pupa stage ; C, 

 adult protruding from the abdomen of a 

 crab. 



is glued by the secretion of 



